Actually, you can use this unit system (called Geometrized units [1]) for anything, but it is especially convenient for describing black holes and general relativity in general. As a rule of thumb, in the geometrized units, 1 solar mass = ~1.5 km = ~5 microseconds. They have the same units so you can use them interchangeably (Our sun weights 5 microseconds).

I'm just being pedantic here obviously..

[1] http://en.wikipedia.org/wiki/Geometrized_unit_system

Actually, black holes density decreases linearly as their mass increases. So, there could easily be a black hole with a density of the sun with supper massive black holes having a density near that of water.

(Where density = mass / volume inside event horizon). Because Gravity ~= Mass/distance ^ 2 but volume is 4/3 pi * r ^ 3