Blister is usually good, because it's the body's way of protecting the damage whilst it heals. However if, as Spindle says, it's much bigger than a 50p coin (which you said it is), then you should get it looked at PDQ.
Definitely don't put anything on it. You run a very real risk of either cooking or infecting it if you do, and if you do go to A&E, they'll probably have to clean off whatever it is before they can treat it.
You could phone NHS direct. You'll be able to speak to a professional (usually a nurse) who will tell you whether you should go to A&E. Honestly, the A&E staff won't mind you going.