Hello There are so many causes of blister formation in old age , these includes. 1 Diabetes , diagnosis can be confirmed by blood sugar level examination . 2 Cellulitis or carbuncle , as this is most common in old age group patient . Usually staphylococcal infection is the main cause of these blisters . 3 Impede supply of blood is the another most common cause , diagnosis can be confirmed by arteriography of both legs ( as you mentioned that it is also in 2nd leg or foot ) . If these blisters which turned into bluish coloration is cause of concern because blue color or dark color means the tissues are dead and blood supply stopped . In my opinion consult a physician and get his opinion. Good luck.