Does this look like a blood blister or something more serious?

Hi - I've just this morning noticed this on the back of my heel. Wasn't there before the weekend. Not sure if the start of something serious or just a burst blood blister etc from poor fitting shoes. [image(https://community.patient.info/uploads/default/upload-sm/upload-sm/1220334-636789328307874067.jpg)

what was it? there is no pic.