I'm raising/keeping blackworms in an unheated, 2.5 gallon tank with a small internal filter for aeration.
I feed them old Indian Almond Leaves (which they are making short work of) and mulm from my established tanks. Occasionally I will sprinkle in some fish food. I do a 50% water change a couple of times a week.
I don't know if my colony is actually growing, but there's still plenty of blackworms sifting through the substrate.
To farm them, I simply scoop out some gravel using my hands and then place it into a container of fresh de-chlorinated water. I swish the container back and forth until the worms are separated from the substrate and then pour these out into another container for feeding.
Any leftover gravel is simply thrown back into the tank.
Hope that helps some. My set-up saves me buying worms and not knowing what conditions they have been stored in, and it's been going for a month or so now. Eventually I plan to have a proper set-up of both these and brine shrimp so I can provide a constant source of live food for my bettas.