I doubt it runs in the lines or even in the whites. To my thinking, it shouldn't be anything different than if someone accidentally got a white bird from their pair of blacks. Just a different color. The other white chick is doing great, growing fairly fast. The one that passed, since I don't know the background of the birds, I can only assume the parents were too closely related or it was just a fluke. I've never seen a blind chick (that was natural without Vitiligo) so I have a feeling this one was one of those things that comes out of the left field.
It's possible it's in the line, but since all of the crosses and pures are coming out without blindness...but it could be that I haven't hatched enough chicks yet either?