In Asia, live food is typically used and discus in these grow-out tanks feed constantly. The Internet is full of websites filled with incredibly complex beef heart recipes, which incorporate shrimp, fish, flakes, Spirulina powder, vitamins, garlic, etc. All the other foods are fed separately, to simplify food preparation and provide some diversion for the fish. Tremendous controversy exists with feeding live food. Tubifex worms have been implicated in a variety of discus epidemics.
Many books also warn against black worms.
Although I do not use either in my tanks, not all black worms are problematic. It is certainly possible to obtain clean sources of black worms, but its up to the aquarist to ascertain the source. Food scientists will tell you that freeze dried and frozen worms cannot transmit disease, despite all the warnings on the Internet. To avoid any possibility of introducing disease, I supplement their diet with red wriggler worms and white worms from my own cultures. However, juvenile discus should not be kept in overly large tanks.