I found this little article.... hope it helps
I'm looking at a Quarter Horse cross who is about 12 years old and appears to be perfectly healthy. My only concern is that he has two blue eyes. I have not had a pre-purchase vet exam done yet due to the vet's busy schedule, but was wondering if there is possibly a vision problem with a horse that has two blue eyes.
If this horse was born with irises that are blue, there is no specific ocular reason not to purchase this horse. Horses with light-colored irises are not at an increased risk for eye disease or injury, and are thought to have normal vision. However, some horses, especially if they have no pigment in the back of the eye (ocular fundus), might squint (blepharospasm) mildly in bright sunlight. I would recommend that you have your veterinarian check the eyes carefully to ensure that there are no ocular diseases present to explain the blue irises (i.e., chronic uveitis) and that the eyes are healthy.[/quote]