Blindsight is described as using your other senses to detect your surroundings. Now, if someone invisible walked into my blindsight zone, would I be able to detect him, because he is only hidden from sight?
PHB p. 291 description of the Invisible condition states in part:
An invisible creature is impossible to see without the aid of magic or a special sense.
Blindsight is such a "special sense". (Other special senses that work against invisible creatures are Tremorsense and Truesight.)
Blindsight is the use of a sense other than sight such as hearing(echolocation), touch(sensitivity to vibrations) to detect things within its radius. It can see anything that comes within its radius.
Tremorsense is a similar ability to Blindsight as well, but is limited to those who are touching the ground within a certain radius of the character.
All of the above forms of sight penetrate invisibility.