It is more than just theoretical. I must disagree and confirm that black holes have somehow been experimentally observed.

The only way to generate the wave we observed comes from the collision of two black holes (or neutron stars). We have no other astrophysical object in our catalogue capable to be responsible for such observations. Of course, the observation could be due to something we don't know, but which mimics a black hole very very well (then let's call it a black hole in the meantime).

Concerning the speed of light, the neutron star events proved that gravitational waves travel at the speed of light, like photons and gluons by the way.