These are the bluest, hottest, and most luminous of all stars. They have large masses and low densities. Stars this hot burn extremely hard and run out of fuel in a few million years, so they tend to be extremely rare.
Rigel of the constellation Orion is a blue supergiant.
Supergiants are pretty much gas with very little solid matter so, despite their enormous size, they tend to be surprisingly low mass.
Although they're so rare, being extremely large (and thus having a large surface area) and hot, they're the most luminous stars and are responsible for the blue tint of many galaxies, including our own.
These stars were formerly high-mass main sequence stars.