I do try to fortify them, but I think I'd call it gutloading rather than dusting. I mix a pinch of calcium and vitamin supplements into water, then add this to the worms being stored in the fridge. When I go to use the worms (the next day usually), I rinse them before using. I am hoping that some of the calcium and vitamins are absorbed by the blackworms. Note that I have no evidence to support this actually working!