A sunburn occurs when the body is exposed too much to UV rays from the sun. It may affect any parts of the body, including the feet. It would take several hours to one day for blisters to develop on your feet. They tend to appear as white, small bumps filled with fluids, which can be itchy and painful. You may also notice swelling and redness in severe sunburn. In most cases, blisters caused by sunburn tend to heal on their own. You can speed up healing and reduce discomfort by taking OTC painkillers, applying a cold compress, taking a cool shower, applying aloe gels, and avoiding direct sunlight.