Couple reasons why this can't be the missing mass
1) Everyone suspected every galaxy had a supermassive black hole at its center, we just couldn't directly observe many of them
2) The behavior of galaxies indicates that the missing mass (assuming Newtonian gravity, not MOND) is not concentrated at the center, but far away from it. Unless the big bang created a huge huge huge amount of tiny black holes they can't be responsible for the missing mass.
Personally my money's on theories needing to be revisited. Not necessarily MOND, but something that indicates our calculations are off. Given that we not only have dark matter but also dark energy as fudge factors now, and can't identify either. Or maybe that's just what I hope, because I don't want Einstein's theories and the standard model to be it, with only quantum gravity as the last unknown. I want Douglas Adams to be right, and once we figure out the universe it is replaced by something even stranger. Bring on supersymmetry, hidden dimensions, hell at this point I'd even take Electric Universe just so things don't get boring!