I don't think it was the spell. It was implied that the spell was forbidden not because it would lead to loss of sight but because failure of the spell was high and as such you would easily lose yourself in the Lifestream. Being lost in the aether led to being blind and as Y'shtola says, it was but a small price to pay to spend time in it. The long time she spent in the Lifestream seems to be the reason why she lost her sight.
