An accretion disk is not a black hole. Current physics have no other explanation for the disks, but it does not mean that the black holes are really there.
There is also some lack of precision on the LIGO experiment that does not invalidate all reasonable alternative explanations.
As people already said, the evident is overwhelming on the side of black holes existing. But we don't have any definitive measurement of them.
I think this is as definitive a proof we'll get until we get close enough to directly measure Hawking radiation. Of course, by that time we should also be close enough to "see" the black hole directly...
It's a bit a matter of definition or interpretation but I would count the LIGO observations as direct observations of black holes. But as others pointed out there's lots of evidence for their existence.
Isn't it still the case that they have not been experimentally confirmed to exist, and all talk of discoveries is just conceptual?