The "negative" in this text means a "No" vote on Scottish independence. To "dress up a negative" is to emphasize the good qualities of what is otherwise a negative or unpleasant thing. The author is saying that it is hard to cast the "No" result of the referendum as anything other than a rejection of the argument for independence; in other words, it's just a "No" and there's not much that one can do to take the sting out of it.