Black holes with event horizons are most definatly black (well not purely black if you take into account Hawking Radiation, but lets not start talking Quantum, because im not good at it
). Light can not escape therefore, black is the only colour they could be.
Dark Matter is called such, not because we cant prove it, only because we cant see it yet we can observe its effect on the 'light reflecting matter' in the universe.
These things are definatly not fudge factors, they are real tangible and observable phenomena