Corys will indeed love blackworms, and they also enjoy frozen bloodworms if you partially thaw a wad of them and allow that wad to sink. Some people use a shallow shell or some other kind of dish to contain the cory food for them, but mine are extremely good at finding what they need. They'll eat any kind of sinking pellet, including the veggie ones for plecos. It is easy to overfeed your tank and create problems when trying to feed corys specifically, so watch carefully and be sure they are getting it all.