Can a genetically true blue roan horse have any white patches on it such as blaze its face?

Yes, a genetically true blue roan horse can have white patches on it, such as a blaze on its face. Roaning is a color pattern in horses caused by an even mixture of white and colored hairs, and it can occur in any color of horse, including blue roan. White markings, such as a blaze, are caused by a lack of pigment in the hair and are not related to the roaning pattern. Therefore, a genetically true blue roan horse can have white patches on it, including a blaze on its face.