If the multicolor penalty didn’t exist I’m confident that Black would be the only monocolor codex that could be considered viable, and even then as Shadow_Night_Black says people would probably use Demon/Finesse/X (though I think Necromancy would fill in the X at least some of the time since Garth’s stuff does have lots of synergy with Demon/Finesse). The problem isn’t the penalty, it’s that Black (especially Demonology with the Black starter) is so oppressive even without multicolor.
I still think that the minimum change needed for Black is to make Deteriorate and Dark Pact both cost 1 to cast. There’s no cost if you play the two together, since Deteriorate’s only cost is to hand size (which Dark Pact covers) and Dark Pact’s only cost is base damage (which is irrelevant in most games unless your opponent is playing for a base race, which very few non-Red codices can do. If either one gets an added gold cost they both have to so that Reputable Newsman can still block both of them, so the natural way to handle it is to make both cost 1. Even with this change I’m sure Black players would still use them both a lot, they just wouldn’t be able to use them quite so oppressively. Black probably still needs a few more tweaks, hence why I said it’s the minimum change needed, and Blue probably needs some buffs (Law in particular), but I consider this to be the most important part.