You'd just use some math to remove the influence of the black hole. Or for a more hands-on answer, you could have some reference beacons that are significantly further away from the black hole.

In a broad sense, the answer goes like this: Almost everything in the galaxy shares an approximate reference frame when it comes to velocity and special relativity. For general relativity and gravity, that's obviously a distortion so it doesn't count.

In the long term, to prevent very minor drift, we can use quasars as a reference.