The poison ivy (or poison oak or poison sumac) rash is the result of your body over-reacting to the allergic urushiol oil in the plant.
If the body really goes nuts, it sends so much internal emergency chemistry to the area that big blisters can form, almost like burn blisters.
The blisters are not full of poison ivy oil: that oil is already under the skin upsetting the immune system. The blisters are mostly filled with watery goo.
But the blisters are ugly as can be, and if they pop open then the site can become infected since the skin surface is opened up.
This level of rash needs medical attention!