Is there redness, tenderness and pain?
Since how long are the symptoms and blister?
This can be a boil or a carbuncle or folliculitis. This would need a course of antibiotic, so it would be prudent to get a script from your physician. Cold compresses and anti-inflammatory analgesics would help.
Please read about folliculitis, boil and carbuncle in these resources;
Having said that, an high index of suspicion should be kept for these symptoms and a physical examination by a doctor is essential to rule out blood clot (deep vein thrombosis). Read about it;
Please feel free for your follow up questions.
Do you have circulation difficulties? A stasis ulcer is what this sounds like. Your doctor will need to take a look at it to confirm what it is. Treatment is keeping it clean and covered, use of compression hosiery, and keeping your legs elevated above heart level as much as possible. Often a certified wound care nurse will be sent to take care of the ulcer in order to improve healing. Open sores have a tendency to become infected, so there are a lot of space age materials that can be used to give any lesion a better chance of healing. You definitely need to see the doctor to rule out other possible causes for this lesion.