this just started since the IVIG? or perhaps you didn’t notice. I would get a doc to look at it. classify what it is, blister, inflammation, lesion etc. dark red or purple tells me inflammation, infection and or clotting. Be cautious to how you feel and go to the doctor. keep drinking.
If there is no sign anywhere else, like a psoriasis or topic dermatitis thing, I be really concerned and get to the doctor. walk in etc. Good luck.