Bloodworms are fairly high in protein and better then brine shrimp. Check the nutritional info on the package.

Whenever you have shy fish like that, try smashing some food into an dead open brain skeleton and leaving it for them to pick at later. Bloodworms work good for this. This is a trick I learned from Instar for shy copper band butterflies. This may not work as well for cardinals because I think they are more of an open water feeder. Also, vary the time of day you feed. Cardinals are nocturnal, night time feeders. Finally, do some research on the feeding habits of a fish before you bring them home.

A fish may feed readily in a tank at the store, but may not feed well with the competitive pressure of a community tank at home. This doesn't sound like your case, but just an observation I have made. I have seen it frequently with the CBB. They may feed well in a tank at the LFS with low flow and not a lot of other fish in the tank, but when you get them home with fish flying all over the place chasing food moving quickly in a reef tank they don't do as well.