I think I get what you're asking.
Blood should be drawn from the line without the cap, not through the cap. It sounds like they're drawing through the cap, then replacing it with a new one.
When I draw through any kind of CVC, I first wash my hands with soap and water, don clean gloves, lay out all my supplies on a clean surface, priming my new cap/extension set with NS. I remove the old cap from the line, flush with 10cc NS, then using the same syringe, draw my waste. Then I quickly attach a new syringe, draw the sample, and quickly attach the new cap/extension set and flush. The blood gets tubed AFTER the line is capped.
Masking and sterile gloves are not necessary. Clean gloves, a clean working surface, and preventing contact of the business ends of the line, the caps, the flushes, etc. are all that are required. The gloves are to protect ME from the patient's blood, not to protect the patient from infection. Preventing all of the connecting parts from touching anything and minimizing the amount of time the line is open to air is how infection is prevented.
Under normal circumstances, cap changes are done weekly, although if the person is getting daily blood draws, I would probably just go ahead and change the cap too, because you're already opening up the line, and I don't know for sure that the end of the old cap has remained sterile during the whole procedure.
For accessing ports and doing dressing changes, you use a mask and sterile gloves/sterile technique.
At least, this is the P&P at my place of employment.