S1ren, that's the best set of instructions I've seen yet for culturing blackworms. But note that even this person admits that the culture doesn't produce a lot of worms. Unfortunately, this is the big drawback to culturing them. If you need any significant quantity, it may work better to just buy them.
One thing I've noticed in tanks that have blackworms living in them: the worms never "go after" flake food or pellets. But if there is a leftover chunk of earthworm, they will invade it - I assume they are eating it. If I were trying to culture them again, I'd feed them an occasional chunk of worm rather than fish food.
Good luck, and keep us posted!