Are blue eyes common in horses?

You failed to mention if the spots have just occured or has he had them for a long time. A brown-eyed horse with blue spots, especially if just recent, should be seen by a vet. Just like people, horses can have different colored eyes. The most common being brown, then blue and lastly amber. I have seen horses that have one brown and one blue. But I would have a chat with my vet just to make sure your horse is not suffering from a serious eye problem.