Could a black hole's dark matter content be determined? I recently attended a talk where the speaker said that it is impossible for us gain information about the history of particles which make up the black hole. i.e we won't know if the black hole was formed due to huge lump of hydrogen or if it was due diamonds. I not quite sure about how will it conceal the information about the charge of particle. Can you explain?
But my main question is that- Is the above principle(?!) applicable only to standard model particles or is it a general one for all particles interacting through gravity?
Will Dark Matter follow it? If no, Can we somehow distinguish it form the rest?
 A: The "no hair conjecture" has not been proven rigorously, but most physicists would say that a black hole is fully described by its mass-energy, linear and angular momentum, position, and electric charge. (So yes, the total charge of what you made the hole from will be visible. )
However, there are various hypothetical kinds of fields that would add extra "hair types" if they existed, and of course dark matter might have such properties. It is just that there is no evidence for that yet.
A: In GR, we have the no-hair theorems, which state that all stationary black holes are equivalent except for their mass, spin, and electric charge.

But my main question is that- Is the above principle(?!) applicable only to standard model particles or is it a general one for all particles interacting through gravity?

The theorem is classical, whereas the standard model is quantum-mechanical. The theorem holds for electrovac solutions, which means solutions to the Einstein field equations in which the only fields are electromagnetic. For other fields, solutions with "hair" are known.
There is ongoing research on no-hair theorems in the presence of other fields besides the EM field, so the answer to your question may not be known. However, the impression I get is that we do expect the no-hair theorems to hold more generally for astrophysical black holes, so that information about lepton number, baryon number, and so on are completely lost in the formation of a black hole.
