Black worms blood worms and tubifex worms are all separate types.
Black worms look like mini-sized earthworms, are the easiest to keep, and IMHO are the healthiest.
me personally I generally keep 3-4 portions (3-4 ounces of worms) at a time in a shallow tupperware container with about 1-2mm of water thats changed everyday. IME they never stay in the water anyway I always find the partially attached to the side of the container. They just require a lot of oxygenation so make sure the rinse them with either aged or dechlorinated (or tank water) water everyday. Oh I keep mine underneath my aquarium in the stand and have had no problems with that although they do stay longer in the fridge (months).