By DCist contributor Abby Lavin Fans may know Keith Thompson as “Sinister 6000,” “Fly Ricky the Wine Taster,” “Funk Igniter Plus,” or another one of his 50-odd stage personas. But for his show last night at the Black Cat, he was billed simply as “Dr. Octagon,” Thompson’s most critically acclaimed alter-ego. Confused yet? As it turned out, the Bronx-born MC came to the stage not as Dr. Octagon, but as Kool Keith, his original…