Friedrich Wilhelm Bessel, a German
astronomer, discovered the first blackhole in 1884.
Every single star in the sky moves, but since they are millions of lightyears away from us they don't look like they are moving. Mr. Wilhelm Bessel studied the motion of stars closely, because when we move around the sun we change position so we see the stars at different angles all the time. Because we move we see the star follow a slightly wiggly path instead of a straight line.
If the star is closer its path seems a little more uneven. If that wiggle in the star's path is measured with particular care the distance the star is from the Earth can be figured out. In 1838 Mr. Wilhelm Bessel became the first person in history to ever calculate the distance of a star.
When Mr. Bessel was following the path of Sirius, the closest star to Earth, he saw that it was orbiting around something like Earth orbits around the sun. Mr. Bessel figured out that Sirius was about two times the size of our sun, so it must be held by the gravitational pull of something about the size of a star. He decided that Sirius had a companion star that he couldn't see and that the two stars were circling each other. Mr. Wilhelm Bessel then decided that Sirius's companion star was one that had used up all of its hydrogen and burnt to cinders a long time ago and since it had used all of its hydrogen it didn't shine anymore and therefore could not be seen. Those burnt stars are what we now call Blackholes.