I'd think the sight thing may tend to make her more nervous at first, but as she gets used to it she should be fine. I don't think it should make much difference. I knew a woman with a blind horse who did trail riding with her. She didn't really do much different, besides telling her to "step up" when asking her to step over something or on top of something during rides.
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.