Haven't you seen the documentary Daredevil?

I'm guessing they maybe have fully blind players, and partially blind players. The partially blind have to wear masks.

Just a guess though.

Edit: so actually according to Wikipedia:

Teams are permitted to use sighted athletes as goalkeepers and guides;

All players, except for the goalkeeper, are blindfolded.

There are leagues where partially blind people are allowed to play, they're ranked by level of impairment and can have x number of players at higher vision ratings.