Three 6 Mafia's Crunchy Black -- Pleads Guilty in Meth Mix-Up

Former Three 6 Mafia rapper Crunchy Black will have plenty of time to study the difference between cocaine and meth -- he's gonna spend the next 7 months in jail for his Vegas drug bust.

CB was in court Wednesday and pled guilty to misdemeanor drug possession. TMZ broke the story ... Crunchy was arrested earlier this month for meth.

Crunch had the worst defense we've heard in a while. According to the police report, when an officer found the drugs on Crunchy ... he replied, "Oh damn. I was trying to buy that powder, not meth."

The Oscar winner got 30 days for the drugs, but also had 6 months to serve for a domestic violence case he'd skipped out on -- so, he'll do the time consecutively.