Blisters are common. Everyone has experienced a blister at some point or another. They can be painful, uncomfortable and annoying. There are several causes for blisters. So called burning blisters are caused by extreme heating or cooling of the skin. Friction blisters are caused by pressure, pinching or rubbing. Furthermore there are numerous skin conditions which can cause blistering, e.g. sunburns or insect bites. Your question however, relates to blisters on the feet so it is safe to say these are caused by friction.
Why do these blisters occur? A friction blister is a soft pocket of raised skin filled with clear fluid caused by irritation from continuous rubbing or pressure. This type of irritation causes minor damage to the skin and the tissue just beneath the skin, then fluid accumulates just beneath the outermost layer of skin. Some of us are more sensitive and suseptible to blisters than others, depending on the thickness and strength of our skin. Heat and humidity increase and accelerate the risk of blistering. So when the feet are all warmed up, they become more swollen and could make your ‘perfect’ fitting shoes all of a sudden feel smaller and tighter, causing friction and blisters appear.
Rubbing seams in sock or shoes, sand and grit, ill-fitting shoes, or non-leather footwear (e.g. plastic) where moisture can’t escape, are all causes for blisters. Even your toes rubbing together can cause blisters.