Blackworms instinctively go to the bottom as they will burrow down into the substrate. I have a tank set-up for them and they spend their days in the substrate eating old Indian Almond Leaves.
To feed them, I simply rinse them off using dechlorinated water and use tweezers to pick out individual worms for my fish. Rinsing is important if you store blackworms in the fridge, as it's not really a healthy environment for them, and the water can foul up quite easily.
I also always rinse new arrivals to the 'blackworm colony' as you don't know what conditions they were housed in, and they have been known to introduce disease into aquariums.