Blockchain just means every transaction is recorded forever (simplifying a little bit but not too much). You can have a blockchain currency without any mechanism to exchange / trade / transfer it.

There are also even regular cryptocurrencies that can only be traded within a ‘walled garden’ where the house always gets a cut. But I don’t think they’d allow P2P sales, ever, and going by GTA V, probably won’t allow P2P cash transfer at all either.