I'm not really sure I understand what downvoting is useful for. The site rules say to blacklist and move on if you see something you don't like, so being able to downvote seems counterintuitive to that. And it isn't for flagging MS Paint doodles or stolen artwork obviously that's what the report button it for. And a downvote doesn't provide any constructve criticism if there is something off about someone's art. So what point is there to being able to downvote?
Updated by NotMeNotYou