I am usually pretty careful about avoiding poison ivy habitats at all seasons. But the last time I erred, I discovered a wonderful dual product that (a) will prevent blisters from developing or (2) if the blisters have developed, will relieve the itching and dry the blisters within a few days.
It is called Tecnu (the preventive)/Calagel (the curative). I bought it at the Rite-Aid in Williston.
I now carry a small bottle of Tecnu in my pack at all times.