Yes, a breeder should be knowledgeable about the colors, but in their defense, I have seen a few grays that bore a striking resemblance to a blue roan. We had one in for training for a while that I would have sworn was blue roan had I not known his breeding and known that he was gray in advance. He didn't start showing any gray on his face/head until he was almost 4. His head, legs, mane, and tail were all jet black with a roany looking body.
Because you said the stud has one gray and one non-gray parent, then he is heterozygous for gray. That means that your colt has a 50% chance of going gray.