Is everyone in Bloodborne blind?

The answer is... kinda in terms of lore, definitely not in terms of mechanics.

To answer the latter first, the combat relies on line-of-sight. Take the regular hunters you fight early in the game: if you walk around a corner and a hunter sees you, he'll attack. If you walk slowly behind one he won't notice you and you can stab him in the back. They even have lines indicating, if not sight directly, the concept of "looking" for the player. ("Found you!", "You can't hide for long!", etc.). You can even look at close screenshots of hunters without their early clothing and you can see they have relatively normal eyes.

Hunters that you fight who have the same move pool as you also tend to wear eye-coverings, but not always. One guy wears glasses, which doesn't make much sense for a blind person.

Even outside of the humans, plenty of monsters play around with the concept of seeing the player.

The Winter Lantern enemy attacks by looking at you.

The Brain of Mensis looks directly at the player and sees you move your body in a certain gesture.

Ludwig's mid-fight cutscene shows his eye move and focus like a normal eye, albeit after beating some sanity into him.

Plus, your character and other people can read and write notes on pieces of paper. And the entire gesture system would be kind of pointless if not for sight.

All of this seems to indicate that the eyeless look is more of a cultural dress style than a practical concern.

As for a quick note on the lore reasons why people dress in a way that seems to disable or remove the eyes, that would require almost a full write-up of the game's story. Short answer is, the covering of eyes doesn't have any gameplay modifications that causes people to act blind, it only provides story and motivational information for characters.