In General Relativity so called geometrized units are very common. Especially the ones where the speed of light and the gravitational constant are set to unity $(c=G=1$$)_{\mathrm{GU}}$. In such a unit system one can measure basically everything in powers of one base dimension; often length.
In this setting masses have the same units/dimensions as lengths. So I would say $M$ refers to the gravitational mass of the blackhole. One solar mass $M_\odot=1.9884\times10^{30}\mathrm{kg}$ is equal to $1476.57\mathrm{m}$ in gravitational units. The conversion factor is $7.42592\times 10^{^-28}\mathrm{m}/\mathrm{kg}=(G/c^2)_{\mathrm{SI}}$.