You don't get Black Purebred TB's. And every horse registered in the Wetherby's Stud Book will be listed as Brown and not Black. They may appear black but they are really a Muddy Black - have a brown-ish winter coat or brown tnged summer coat, get brown bits in the mane and tail, brown cat hairs, browning around the eyes and muzzle. To all outwards appearances they are predominantly black.
With outcrossings now, and thus Non-Tb's, you can get true blacks but they will always be registered as Brown.