A few points for your consideration:
First, this blast on December 28, 2009 that killed over 50 people should be seen as part of the ongoing wave of violence against Shia Muslims in different parts of the country, including D.I. Khan, Hangu, Peshawar, and Parachinar. Hundreds have lost their lives in these attacks. They also are linked to broader wave of violence and blasts that have been going on throughout the country for the last couple of years.
Second, it was nothing less than a systematic and careful plot. Many local and international players were probably involved. Just a few days ago, four Jundallah members were captured by Karachi police, which claimed that the members confessed their involvement in the three Muharram blasts in Karachi this year, including the Ashura blast. Jundallah is widely known for its connection to the US (see an acknowledgment in ABC News). Jundallah is also known for its connection to local extremist-militant groups in Pakistan.
However, one should also take the Police's claim with a grain of salt, given their silence on how exactly were these members involved, and given the way the Police works in Pakistan: It is not unimaginable for the Police that under the pressure of their superiors or public demanding justice and efficiency they would capture innocent people or petty criminals and charge them for a high-profile crime.
It is hard to tell the reality of those captured from outside. It won't be surprising if those captured were really not responsible for the blast. But that should not invalidate the general statement that various interests were probably behind this blast (which I argue further below). This also should not invalidate the fact that since the 'Afghan Jihad' many extremist-militant groups in Pakistan have more or less remained connected to and dependent on Pakistan's security agencies, and some of them directly to foreign actors (like in the case of Jundallah). Their involvement in “sectarian” violence in the past is no secret. Time and again, they have done their master's bidding. Jundallah, for example, claimed responsibility of the bomb blast in the Iran-Pakistan border area in October 2009 that killed more than 40 people including Iranian military officials and local tribal elders – Sunni and Shia both.
Third, consider also the systematic nature of arson and torching of shops and buildings in the particular vicinity (Bolton Market) by masked miscreants who were all dressed in black and came prepared with cutters and inflammable chemicals. All of this was captured by security cameras (see Urdu clip from Dunya News). It is hard to imagine that they could be the Azadars (participants of the Muharram processions) with those accessories ready who somehow knew beforehand that a blast would happen and they would have the opportunity to steal and torch those shops.
Consider also that the "reactionary violence" by "angry protesters" - as some news sources framed it - happened only in the Bolton Market. If the reactionary violence was indeed caused by irrational impulses of angry protesters, then the systematic arson and property destruction should have happened at multiple points in the miles long procession on the M. A. Jinnah road. The plotters surely screwed up this part in their planning.
The blast and its aftermath were clearly well-planned, perhaps coordinated among multiple groups whose diverse interests all converged into making it happen in Karachi.
Fourth, to frame these attacks as "sectarian" in the news misleads an uninitiated viewer into believing that this is a fight between the Sunnis and Shias in Pakistan. That is not true. For one, consider the background of people killed in the Ashura blast: according to one report 15 of the 50 killed were Sunnis, two Bohra Shias, and one Christian. Anyone familiar with the ground reality in Pakistan knows that Muharram processions are widely attended by Muslims from diverse sectarian background, and at some places, even non-Muslims also participate. Thus the processions are not an exclusive tradition of Shias, contrary to how it is framed in the international news media. They are for all those who want to commemorate the noble sacrifice of the grandson of the Holy Prophet, Imam Hussain. However, the reductive representation in the international news media further distorts the image of Sunni-Shia differences in the mind of general viewers: An attack on Muharram procession is automatically seen as an attack on "Shias by Sunnis" in news and analysis. Perhaps that was also the intention of the perpetrators of these attacks: to provoke sectarian differences and to distance Sunnis from Shias (and Muharram processions).
This is not a fight between Sunnis and Shias in Pakistan. The ongoing wave of terrorist attacks is aimed at not only Shias but also those Sunnis who are against extremism or who do not fit into the strategic equations of the bigger players in this game. Maulana Sarfraz Ahmed Naemi who was killed in a suicide attack on June 12, 2009 is a case in point. (For the role of the Taliban in Pakistan and how their various groups intentionally or unintentionally play into the hands of bigger players, see here).
Fifth, the Ashura blast should also be seen in relation to the "ethnic" clashes among different groups that are going on in Karachi for many months now. The clashes are partly about political control against the fear of a demographic shift (with the continuous influx of "migrants" from northern Pakistan (and interior Punjab) areas) and partly about land-grabbing interests. In the past, concerns were raised by different groups that a certain ethnic group may be targeted in the name of curbing "Talibanization" in Karachi. That, all it would require is a provocation - from whatever side it may come from and in whatever name it may be carried out (sectarian or otherwise) to spark clashes. In response, others contended that there was indeed a rise of extremist-militant activities in certain neighborhoods in Karachi and the "Talibanization" threat was real and imminent.
In one broader perspective, the purpose of planned violence all throughout the country is to pressurize the civilian government or the Army to give in to certain demands, and for that reason, Blackwater's role should also be considered in any investigation (See Blackwater in Pakistan: Loose End or Larger Strategy). Another perspective suggests that the volatile conditions may further weaken the central government and destroy its political alliances, paving the way for a sweeping political change.
Sixth, in the global context, one should also question the simultaneous escalation in "sectarian" violence in Iraq and Pakistan. Political analysts argue that instability in Pakistan, Iraq, and particularly Lebanon are key indicators of possibly a decisive move by the US or Israel against Iran and/or Palestinian resistance. Instability in the case of Pakistan and Iraq would lead to further chaos, so their populations would be kept busy in themselves. For Lebanon, it may be internal strife among different militia groups or an attempt to 'neutralize' the resistance movement so that it cannot respond to any major Israeli or US aggression against Palestine or Iran.
One should carefully follow the developments on the international scene, including the media's role in manufacturing consent for wars. In the last couple of days, front page headlines on the line that 'US arms gulf states against Iran attack' were seen on all major news sources in the US and UK (See, for example, NYTimes, WSJ, Telegraph). Recently, General David Petraeus, who heads the US Central Command overseeing US forces in the Middle East, Gulf, and Central Asia, has repeatedly made provocative statements about bombing Iran. However, this also could be just psyops to pressurize Iran. On a related note, on Jan 18, 2010, the Israeli newspaper, Ha'aretz reported that the Turkish Prime Minister, Recep Tayyip Erdogan warned Lebanon that "Israel may be planning attack". A statement like this to come from Turkey is not very usual.
Within the context of these reports, the point here is to keep an eye out for political developments at the international level, as they will certainly have an impact on politics at the national and local levels in concerned countries.
Commenting on the Indian reaction to the Mumbai attacks in November 2008, the Pulitzer Prize winner author, Arundhati Roy wrote in her characteristic perceptive style (Guardian, Dec 12, 2008): "Almost always, when these stories unspool, they reveal a complicated global network of foot soldiers, trainers, recruiters, middlemen and undercover intelligence and counter-intelligence operatives working not just on both sides of the India-Pakistan border, but in several countries simultaneously. In today's world, trying to pin down the provenance of a terrorist strike and isolate it within the borders of a single nation state is very much like trying to pin down the provenance of corporate money. It's almost impossible."
Building on the same premise, the above talking points draw attention to the multitude of players (rather than simply the "Taliban"). All evidence suggest that the Ashura blast should not be seen as an isolated occurrence and cannot be simply explained through "mindless fanaticism" of the "Taliban" (without tracing the hands behind them). Moreover, these talking points raise the question of interests and ends: Who benefits? And, how do such tragic events fit in the broader equations of different political players?