If it was a true blue it would have been pretty expensive.
Funny, Ive never been stung by carpets (or any anemone), or euphyllia, even when I used to handle them when I worked at a lfs. My coworker there brushed his arm against something nasty in a tank, he cant remember what did it, and ended up with a rash kinda like yours, covering most of his forearm, its still kinda there and it happened half a year ago maybe.
Bet it feels good when you dip it in saltwater