I don't think anyone knows why he blinded them. But it involved some kind of spell that is native to his world, and he needed sacrifices.

[There's no disguising the disgust and anger in his voice.] Perhaps he blinded them because he did not want them to see him. He kidnapped them over a period of time; he would not have wanted his cover blown.

Or perhaps he wanted them to feel a particular emotion. The victims were drained of their emotion to be used as materials for the ritual.