If you think coins can be blacklisted then you are very wrong. We already have mixers and if someone tries to "blacklist coins" there will be lots of services that will appear to help with that.
Blending a pound of feces with a pound of pasta doesn't give you a kilogram of anything remotely edible.
What do you expect will come out of these mixers? Whitelisted coins?
You missed the main point. Sending money to MP means losing possession of those money and GPG will not stop MP to run away it them, while a smart contract will make it impossible for anyone to not fulfill the contract since the coins will be locked and will be released only when the contract conditions are met.
So, how would you actually, in practice, create such a, ahem, "smart contract" ?