Is there a reason to rule out the possibility of material becoming Dark Matter after falling into a Black Hole ?
In respect to black holes, like most things, there is more than one way to skin a cat ( what all the ways have in common is the cat doesn't like any of them ). With the classical black hole, everything inside the event horizon is speculation. Even equations used to give answers about the conditions inside the event horizon cannot be compared with reality by observation.
On to skinning the cat a different way. The following is in my two cent opinion (IMTCO).
The following is dropping out consideration of dark energy and dark matter except with black holes.
If a nebula is a fair example of the universe shortly after the microwave background radiation era, than a star's photo sphere would have similar conditions to those at the microwave background. A neutron star has a lot in common with the early universe main features. (Consisting of neutrons, photons, exotic particles and photons.) Black holes would represent an even younger universe consisting photons or dark matter or dark energy.
PS. I haven't read any reason why black holes couldn't consist of photons or dark matter or positively curved dark energy.