Those black markets were for food and other essentials though, which can't really be banned without the entire population starving (although they damn well tried). Monero might be useful in a world that doesn't respect property rights, but maybe not even then. In a totalitarian state, anyone can be a spy of the regime. Think 1984. So you buy something with your monero from a vendor and boom, you're dead.
Discussion
Not sure how it being food and other essentials makes it any less relevant. It was still black market. It was not just those things either. It was music, clothes, tools, parts, and much more. Basically anything that had demand.
Maybe, but in that case Bitcoin would be even more useless because it is much easier to spy on
They can't even keep drugs, phones, etc from entering prisons - the most locked down, surveilled, and controlled populations on the planet. I think we'll be ok...