After reading up on Enrico Dandolo, 42nd Doge of Venice, I had a few questions that I hope someone could answer.

Enrico Dandolo was said to be already blind during his appointment to the position as Doge of Venice. Was this common, and did this influence his function as Doge? He also took part in the fourth crusade, and funded it if I understood correctly. How would a blind man contribute to the Crusader, and why would he? If I understood correctly Venice and the Byzantine Empire were not on the greatest terms. What was Venice's part in the sacking of Constantinople? Its sometimes attributed as part of a deliberate scheme of the Venetians, but is this accurate?