I've tried a few methods for keeping blackworms, including some fairly elaborate culture tanks with various specialized conditions. If you aren't ready to set up a large volume of water, I think it's more cost effective (both in terms of time and money) to pick them up at a local shop. If you find something that works, be sure to share it. There isn't much good info online. (lab conditions with paper towels are NO way to grow blackworms)
Not that it would be practical for everyone, but a medium or larger sized top-feeding predator tank is an excellent environment to grow blackworms. My 75 gallon (hosting 1 medium sized arowana) is full of them. I don't think I've purchased any in months, but the tank has thousands each time I harvest (typically weekly). I've seen the arowana grab them in the water column, but it doesn't seem to care about them in the pea gravel (covering a large portion of the substrate) at all. Living with a large top feeder the worms have a great opportunity to not be eaten and a continual supply of excess nutrients in the tank to encourage growth. Also, most predator tanks necessitate good filtration and somewhat regular water changes, (although I'm a bit lax with changes), so the conditions are good for production.
The fact that the worms proliferate by segmentation also helps. The act of harvesting seems to help the population increase. For me, pea gravel (~5mm) works well as a substrate. They can anchor to it easily and don't have a hard time digging in quickly to avoid predation. When scooped through with a net for harvesting (I basically run a net through the gravel a bit to move them into the water column and then scoop them out with the net, usually while I'm vacuuming the substrate) the disturbance seems to cause more segmentation and yield. You can visibly see a carpet of worms in the substrate of the tank, with higher concentrations in the areas under which feeding regularly occurs.
Again, not a great solution unless you happen to want an arowana (or something similar) and also keep some blackworms around for smaller fish...but it's a data point...
