"Blinding" was a gruesome but recurring punishment in the Byzantine-influenced Mediterranean. It was, as you might ascertain, punishment for very serious crimes.

I narrated a bit of the early politics of the Venetian Republic in this answer which you might be interested in. Numerous early Doges, backed by their supporters, tried to elevate their sons as co-rulers destined to inherit full powers once they died, which antagonized leaders of rival factions resulting in tumultuous successions: Maurizio Galbaio elevated his son Giovanni as co-ruler in 778, however Giovanni had to flee for his life in 804 when the people (or rather, wealthy males —let's not forget Venetians' notion of "Republic" is much different than out own—) backed Obelerio Anafesto’s plot to murder him after he threw the Patriarch of Aquileia off a tower for refusing to consecrate his cousin Cristopher as Bishop of Torcello (long story).

That same Obelerio was then elected Doge, however he too soon inspired the ire of the people: a risky political ploy didn't play out as planned, resulting the Frankish Emperor Pepin attempting to conquer the lagoons by force of arms. Obelerio and the two brothers he had elevated as co-rulers were exiled and one Agnello Partecipazio was elected (incidentally, Agnello Partecipazio was the first doge from the Realtine Islands where modern Venice stands; at the time they were a marginal community).

Agnello elevated his son Giustiniano co-ruler, who then elevated his brother Giovanni as co-ruler. But after his father and brother’s passing, Giovanni proved ineffective at dealing with Dalmatian pirates who hampered Venetian trade in the Adriatic and ended up being forcibly removed to a monastery by an angry mob.

Pietro Tradonico, the chief builder of what was becoming modern Venice on the Rialtine islands, was then elected Doge. Following a call to arms by the Byzantine Empire which wanted to reclaim Sicily from the Abbasid Caliphate, he assembled the first great Venetian War Fleet only to lose everything spectacularly in his first naval engagement. He then hosted Emperor Lothair II in Venice, deciding that maybe it was a better idea to get friendlier with that other Empire. Having managed to piss off both the Pro-Byzantine faction and the Pro-Imperial factions in the city, he was stabbed to death either after or immediately before a riot.

You can understand that the Venetian Republic's complex system of councils and ballots took centuries to develop. In the first few centuries of its existence leadership was very much an ad-hoc affair, with society largely split along three lines: the pro-Byzantine landowners in Eraclea, the pro-Republican merchants of Malamocco, and the pro-Frankish Clergy. These factions, who assembled in rambunctious, sometimes mob-like assembles in order to select their leader, favored strong and popular personalities to govern them; however these same candidates often had undesirable hidden qualities and dynastic ambitions.