My experience with blackworms parallells Paul b's exactly. They are a virtually sure fire food source for tiny 10-20mm butterflies and trunkfish, which prefer them above all things. My blotched anthias was hesitant until he tasted them, and now scarfs them down along with mysis, pods, shrimp, etc. They are a supercharged food that will grow a fish very quickly, and it's the aquarists job to see that a wide variety of other foods are also eaten, fresh if possible.
Of course, the worms must be fresh and healthy. I keep mine in the refrigerator in a double level plactic contraption that allows me to rinse and change the water every day. It takes two minutes. The worms have no odor if cleaned properly. I usually buy enough to last for about 2 months. I've seen very few fish that don't eat blackworms ravenously. Blackworms are found in clean, clear, cold fresh water, not the muddy swamps tubifex worms inhabit.There are several species that I would not have been able to keep successfully with were live blackworms not available. Rinse before feeding.
Years ago, I had a girlfriend spend the summer at my place. She flipped out when she found the worms in back of the veggie crisper. I pointed out that the worms were here before her, and would probably be here after she went back to complete her art history degree.