"Blind" makes it so the targets can be mixed up, but only if they are on line with each other. If Garnet tries to use a spell or an item on Quina, she targets someone on her left and there's only Quina there. If she tries to target Vivi, she knows he's on her right so turns toward her right, but Blind can make her miss Vivi and hit Zidane instead because he's also in that direction.
It's hard to explain with words as it's mostly a visual thing.
Also, it's a system that makes the party's order matter more than before.