Elvis Wasn’t Black means in slang: You respond to losing an argument by making a silly statement, which makes you appear like you’ve been right the whole time. This was born when Brandon stated that Dorian listens only to black music while he listens primarily to rock. Dorian lost the argument and responded by saying “Elvis wasn’t black”. Elvis is well-known as the King of Rock ‘n Roll.
(in Slang Dictionary, added by Zachary Barr)