Despite your precautions a blister has formed ? So what should be done to treat it ?

If the blister is closed and painful :

Pump out the liquid with a small syringe, and inject a drying antiseptic that will clean the wound. A second skin effect dressing is applied (called hydrocolloid) Consult your chiropodist and/or podologist.

If the blister has burst or even split :

Cleanse with a drying antiseptic solution (betadine® or eosin) by cutting around the blister. Then apply a second-skin effect bandage which is protective and healing. 
Consult your chiropodist or podologist.

 Although very often banal, a blister can be very annoying and change the position of your feet. It is considered a wound and should be treated as such. 

People with diabetes should be very careful if blisters form. As their immune system may be more vulnerable, increased vigilance is necessary to avoid complications.