Aren't the stormtroopers clones? how can one be black?
Theories abound, but the prevailing logic is that after the Clone Wars (around the time of A New Hope) when the Empire was peaking, military ranks swelled and new Stormtrooper units were created from the long line of conscripts and volunteers from conquered worlds. Remember, Luke was planning to go to the Imperial military academy before Owen and Beru died. If he had flunked out of pilot school (unlikely) he might have ended up a trooper. The Stormtroopers were basically the rank & file Imperial army by that point. There were a fixed number of clones anyway - Kamino wasn't pumping them out indefinitely - and many of them would have died during the Clone Wars. Some of the EU books postulated that many Stormtroopers were still clones who had lived through the Clone Wars and the fall of the Empire -- Luke could sense this unique quality about them through the Force.