i think it would be fine because there are no exposed wires or mechanism's like say if your waters broke on top of a running hairdryer (although i have no idea how this could happen, unless you were blowdrying the short and curly's!)

plus the liquid would sorta soak into the blanket and not go near the power point or anything like that. i dont think i would worry bout it too much, although i have heard that electric blankets are not great for PG women because they increase the body temp too much.