What was the first black hole candidate to be discovered?
Cygnus X1 is a powerful X-ray source which was discovered in 1964. It is a binary system with one partner being best explained by a black hole. It has an accretion disc of material falling into the black hole which is heated by friction to temperatures where it emits X-rays. It is some 6000 light years away has a mass of about 20 time that of the Sun.
Since then it has been determined that there is a supermassive black hole at the centre of our Milky Way galaxy.
You can see Cygnus X1 in the photo given above as a white dot taken with four different instruments.