So...

...is it possible for planets to form in the accretion disk of a black hole.

The math says: Yes.

Supermassive black holes have huge accretion disks that contain far, far more dust and debris than the much smaller accretion disks that form around stars.

Any such planets would have very interesting orbits indeed: They could be as far as ten light years from the primary. Needless to say there would be no life, at least not of any time we would recognize, on such a world. There would not be enough energy.

At least as far as we know.

But there could be huge numbers of these planets orbiting, say, the black hole at the center of the galaxy. With current technology we can't detect them.

Could they be useful? Perhaps...