What Causes Blister On Lower Leg?

Hello, My mother who is 67 tripped coming down the stairs and banged her lower leg, which has a big bruise and was very swollen. She can walk okay, so we have been applying ice off and on every 20 minutes or so. The swelling has gone down a great deal now, but a small blister about 1/4 of an inch wide has now appeared. What could that be?