First-degree burns do not blister. If your burn has blisters, it is not a first-degree burn. First-degree burns (also known as superficial burns) only affect the outer layer of skin, the epidermis. To cause blisters, a burn must affect the second layer of skin, the dermis.
Sometimes, a burn will initially look like a first-degree burn, but blisters will later form within a few hours. These are also not first-degree burns. It can take a while for blisters to form in some cases, but this does not mean your burn was originally a first-degree burn that later became a second-degree burn. You have a burn that was a second-degree burn all along, but with blisters that were slow to form.
You should seek medical attention if your burn has blisters. Larger burns and second-degree burns affecting joints, the face, hands and feet, or groin and buttocks require emergency care. Do not break or pop any burn blisters; doing so may increase the risk of infection.