Well I think they can but at least in some coins I don't think they would be efective. Let's not forget that the transactions in the blockchain are public, so once they are tracked and associated to a user for some reason, they should be easy to track. A lot of people say that bitcoin is associated with money laundry, but I don't think that's true. Transactions using monero, zcash or private coins like those, might be efficient, but I don't think all blockchain-based transactions make money laundry easy.