Veritasium did video about how no has yet measured the one way speed of light. The conclusion of the video is that we don't really know that the speed of light is constant because we can only measure it when it goes in two directions. It could be that light propagates faster in one direction....that it has a preferred direction. However when you measure the speed of light you have to do it in two directions and its average speed is what turns out to be constant. Interesting video for sure. linked below.
If you found a non rotating black hole could you measure the one way speed of light by pointing a laser toward the non rotating black hole such that the beam followed a boomerang geodesic?