Rancid blackworms are incredibly nasty! I ran into something similar a few weeks back (but luckily I had the blackworms in a beaker and not the tank!)....I almost threw up! Ack. I know doing a complete water change is not ideal but if the water quality is that bad you may want to do that. As Ali suggested, I'd try the gravel cleaner first (you can get them in any pet store). Also, check the filter to make sure there aren't any blackworms lodged in there that will dirty the water again. Either way, I think new water will be better than rancid water. Check the levels of ammonia and nitrites in the tank (you can get this kit in the pet store too, they also check for pH and nitrates).
Once this is cleared up, may I suggest something that had worked incredibly well for me? I put a small feeding dish (you can find those in the reptile section) in the bottom- you can even use a small shallow dish you already have at home. When feeding time comes, I put a few blackworms in the dish and the newts now know that that's where they'll be fed. It prevents the worms from ending up all over the place (although I've had a few worms squiggle out). I only feed the newts what they can eat so there's no left over food around to spoil the water. The newts will get trained to go to the dish and eat as well. It's kind of neat to watch. Good luck and let us know what happened!!