This is not technically speaking impossible, but the Bitcoin community has a visceral hatred for the idea, because it allows off-blockchain-entities to effectively deprive them of the use of their coins. It also undermines trust in the currency generally, since why accept Bitcoins if there is the possibility that you'd be paid with black Bitcoins instead of white Bitcoins, when you're practically speaking guaranteed that all dollars are green.

Also "Take all the coins that went through Silk Road. Now, floodfill forward." (probably) colors most of the network black.