This must be one of the most common questions that people ask. The gondolas black colour originates from a law passed by the Magistrato delle Pompe (the magistracy of the Venetian Republic who was in charge of monitoring the excess of luxury) on the 18th of April 1633, shortly after the end of the plague of 1630.
The motive for passing this law was not because the sombre colour chosen was a sign of mourning, as many would like to believe, but because there was the willingness to standardise the appearance of these boats, imposing not only a uniform colour, but also eliminating many superfluous decorations.
The only exception to the rule was made for the Patriarch (the highest religious authority in Venice) who could use his gondola with a golden felze (cover).