Fracture blisters occur when a bone is fracture which leads to swelling that has no where to go and eventually ends up accumulating in the skin forming a blister. Sometimes when these blisters are seen after trauma, it is very likely to find a fracture on X-ray.
Here’s an example of a woman who had dropped a picture frame on her toe and ended up with a severely swollen toe and blister that formed 3 days later. The toe was found to be fractured after X-rays were taken.
This is a mild example of a fracture blister. More severe ones are commonly seen with ankle fractures and especially fractures of the heel bone.