Speaking as a Byzantine historian, it's difficult to say if Dandolo and Venice deserve the entire blame. Certaintly, Venice had been seeking extensive trading privileges in the Empire and took advantage of the power struggle in Constantinople. At the same time, one cannot blame Latin Christendom or the Papacy, which is ironic since in the 20th century the Pope apologized to the Patriarch in Constantinople (Istanbul) for it. Really, it isn't the Pope's fault, maybe Urban's initial fault for the very idea of Crusading, which was alien to Byzantium. Constantinople had no concept of holy war, as the regulation of war and military affairs was something the Emperor and his generals were in charge of. Priests and bishops advocating it and indeed blessing it were outrageous and strange. Anyways, it isn't the Pope's fault for 1204, he most certainly disapproved and proved it through excommunicating the entire crusading army.
However, one has to examine Byzantium as well. The Byzantines and their squabbling, following not just the decline that began "the day after Basil II died," but also the subsequent corruption of the Angeli and even the brutality of Andronicus the Terrible played a part in setting the stage. It appears even Constantinople's short term thinking and inability to effectively understand the geopolitical landscape between them and Venice was also a contributing factor.
In fact, the very idea of sacking Constantinople was presented by a Byzantine heir, Alexius, son of Isaac who begged the Crusaders (when he was found living in Hungary) to help him regain the throne for his family and remove the usurper. He promised them more than he could obviously deliver and the blind Doge certainly was aware of that. So is it possible that Venice is to blame? Not entirely, it appears it's the largest mistake in history.
That's an extremely short version, since I'm at work. Hope that helps.