When astronomers measure the mass of the black hole, they simply measure the acceleration of nearby stars, using newton's formula. They might use fancy words, but the math is very basic.
Now, if time ran faster, like if you made the measurements while accidentally having the video speed in fast forward. The stars would accelerate faster, and you would get a higher mass.
Now, if time ran faster, like if you made the measurements while accidentally having the video speed in fast forward. The stars would accelerate faster, and you would get a higher mass.