If you are on 4.5 (or the 4.2 downgrade) you can do the 1 time exploit with the blue card. After that you dont need the blue card with you. Unless you launch a DS rom, then it wipes the exploit. However, you can still launch the web exploit if you have internet. Until you get back to your Blue Card.
The only way you can launch a ds rom anyways is with the blue cart right? Or do you mean a physical ds game will wipe it as well.