Visit your local Chemist, and buy some Compeed Second Skin, or Compeed Blister plasters.
Compeed plasters have a special antiseptic fluid built into them, and placed over an existing blister, will keep the bacteria and such like away, allowing the blister to heal without any infection.
I used to keep a set in my bumbag, when out for long walks, and as soon as you felt slight rubbing or discomfort from poor fitting socks, on went the Compeed, and 99% of the time you remained blister free.
The fact you already have a nasty blister, requires some sort of protection from the bugs.
Get some Compeed, and you will be right as rain pretty quickly.