As you said before, that measurement is based on the effect the 'black hole' has on the 'mass' around it. Considering that we are now finding that our 'estimates' of the actual 'mass' ...out there... are not quite correct, then are our estimates of the mass of a black hole incorrect as well?
Earlier this morning, on a thread about FREE ENERGY from WATER (keyword: BLACKLIGHT), it was suggested that 'dark matter' or 'dark energy' was made from Hydrinos. (so 'new' a word that even spell checker doesn't like it).
The best way to sum this all up is that all our 'knowledge' is subject to revision, as we 'learn' more about the Universe.
BTW, the Military turned over operation of some unneeded Spy Satellites which will give us an 'imaging' ability improvement of 100X the power of the Hubble Space Telescope.
Imagine what we will 'learn'.
BTW, here is another thread on the ex-spy satellite.