We get blisters when the skin gets rubbed enough that the epidermis (the skins outer layer) becomes detatched from the fibreous layer beneath it.
This is usually caused by rubbing shoes on the feet or tools in the hands, but can also happen through chemical reaction.
The space beneath the epidermis rapidly fills with fluid to create a cushion, preventing further damage. Therefore, if at all possible, the correct treatment for a blister is to leave it well alone.
If you get a blister because your shoes have been rubbing your heels, take the shoes off. A plaster over the blister will provide padding. If left alone, a blister usually goes away within 48 hours.
Sometimes we need to carry on walking in the same shoes, so in this case the usual treatment is to burst the blister using sterilised needles and then to dress with lots of padding before putting the shoe back on, but this emergency measure is not recommended. This is because bursting the blister leads to an open wound that can get infected, particularly in a warm , moist, smelly environment such as a shoe or boot.
If a blister becomes very red and fills with puss, it is best to get it seen by a medical expert.