I have seen several sites that include blackworms in the betta diet, especially for conditioning for breeding. Try this site:
http://www.splendidfarms.com/Feeding_the_Adults.html
I feed my daughter's betta with the blackworms a couple times a week and the betta acts like a puppie when he sees me come towards his tank. His tank is planted and has some floating stem plants that I put 2 or 3 worms on. This slows their fall to the bottom and the betta gets them easier. He will hunt for them between the plants and rocks as well.