No, it isn't true. Artists like Prince, Musical Youth, Donna Summer, and Tina Turner were all on MTV well before Michael Jackson. I remember Rick James (dammit!) complaining that MTV didn't play black artists because Tina Turner "stopped being black years ago." Oh, Rick.
That idea about Billie Jean being the first video by a black artist on MTV is part of The Gloved One's publicity machine. That machine may rival Elvis' in upcoming decades.