I did a test in PL to see if the black slider has the same effect as moving the black point in the tone curve using a greyscale.
The greyscale has the following equal RGB values:
(0, 25, 51, 76, 102, 127, 153, 178, 204, 229, 255)
Greyscale.tif (1,1 MB)
No corrections applied in PL.
My objective is to adjust the first 3 bars from the left to black, i.e. RGB = (0,0,0)
First adjustment: using Selective Tone, Blacks = -87:
Result:
Values: (0, 0, 0, 29, 58, 88, 120, 153, 187, 220, 255)
Now reset the Selective Tone Black slider to zero.
Second adjustment: using the Tone curve by moving the blackpoint to the right until the 3rd bar has RGB=(0,0,0).
Result:
Values: (0, 0, 0, 31, 63, 95, 128, 159, 192, 222, 255)
The differences between using the black slider and moving the blackpoint is:
(0, 0, 0, 2, 5, 7, 8, 6, 5, 2, 0)
Not a major difference but still a difference.