Not everything relies on vision to sense the world. A creature with blindsight is not affected by darkness or other heavily or lightly obscured areas, and can see through invisibility, within a certain radius. Creatures adapted to the darkness (like bats and moles), creatures without eyes (like purple worms or oozes), and creatures with echolocation or heightened senses, such as true dragons have blindsight.
Creatures that do not rely on visual sight, noted as having blindsight (blind beyond this range), are immune to purely visual illusions (such as those created by minor illusion).