Literally answering your question, you could feed the frozen, thawed bloodworms every day. I would not feed them exclusively though. The bloodworms should/could be made part of a several x a day variety feed.
Your skinny Rosy may actually be a different matter than just getting skinny/loss of appetite from not being an aggressive eater. How old is the Rosy Barb in question and is it a male or female? How long has it been since you started noticing her changing physiology?
It is female and I've had her a couple years. I don't think she's being out competed for food. I watch food float right past her. She just hasn't seemed interested. Nor does she seem bullied or sick. I have only recently noticed the change in her in the past week, but it could well have been gradually happening longer. I'm hoping a few bloodworms feedings will stimulate her.