Actually there was a black 1% MC in SOA, i just forgot about it. IRL did not happen probably because the Hell's Angel's are incredibly powerfull and have lot's of racist members.
Hell's Angels don't dictate what other MCs are allowed to exist, and nor are SOA supposed to be the Hell's Angels. HA are just one MC of many, albeit the most famous and feared of them.
Hunter S. Thompson makes reference to black biker gangs in his non-fiction book Hell's Angels, The Strange and Terrible Saga of the Outlaw Motorcycle Gangs. They definitely existed in the 50s and 60s, and I am sure they still do now.
Incidentally, for fans of SOA, Hell's Angels is an absolute must read. Completely fascinating, hilarious and terrifying book. A real insider's look at the club, Thompson was allowed to immerse himself in the culture and given unprecedented access to the upper echelons of the club, until he was rather unceremoniously "stomped", barely escaping with his life.