One of the quickest, well documented is to use a gray layer and a threshold layer.
Create fill layer, GRAY, blend set to difference
Create threshold layer
Move slider arrow to the left
Slide it back slowly to the right.
The first black pixels that appear are you gray area.
Using the color picker set to 1 pixel mark one of the pixel location
Inactivate both layers (fill and threshold)
Create a level adjustment layer.
Use the mid tone probe, click on marked pixel.
You are done. You can delete both fill and threshold layers.
Please note that while this method helps adjust the black it is far from fool proof as what it does is find the pixels who value is nearest to gray. If you do have any gray you will find an approximation not the real thing. This method works in about 90% of the cases.
I am not an artist. I am a voyeur and a witness.
Creativity is the same as art appreciation: BS as it is in the eye of the 'beholder' and/or 'critic'.