Jo,
Has the blister ruptured? If not, then don't worry about it, it shouldn't become infected. If it has ruptured, clean it, apply an antiseptic and dress it. It should heal (heel!) just fine if you keep it clean. If it continues to be red, sore and tender then your GP or DSN can prescribe a topical antibiotic cream.
MH had an infected toenail last week and the DSN gave him a script for the AB cream, it healed in no time with careful cleaning and regular applications of the cream. It didn't take longer to heal as we were told wounds would with diabetes.
Ju