That sounds like an extremely easy way to get pulled over and busted for possessing a large amount of drugs. I've heard of fake plates but that just sounds loving dumb. Why wouldn't they just take the plate off if they're gonna make it completely black?
I don't know if he meant completely black or not, but sometimes people here use black marker to obscure the actual license number.
Of course, this is an even easier way to get yourself stopped by the police than by drink driving, since the on-board police cameras scan all license plates they see and check them in the DVLA catalogue, and if it doesn't exist it gets flagged and the police car will just stop you.