If you listen to racing people talk, they might call a horse "black type" or talk about the "black type" in a horse's pedigree.
No, this doesn't mean the horse is black or that there's a type of horse called black.
Traditionally, in sales catalogues and advertisements, horses that won or placed in stakes races are bolded (upper case for winners, lower case for horses that have placed). This, of course, makes the type stand out as black.
So, a "black type" horse is a stakes winner or placer.