I have found two Set Curves Point entries in the keymap in the Image section in Blender 2.80.
Only the mask mode provides curves in the image editor. But here the hotkeys does exactly nothing. And we seem to have a key overriding problem here anyways. Since with ctrl + lmb you paint a freehand curve already with Mask add vertex and slide. Also a hotkey only functionality, to find in the keymanager.
Searching in the Blender Manual did not lead to any success. Only the Python API has a thin comment. This seems to be an old functionality, i have found the corresponding entry also in the 2.78 API.
bpy.ops.image.curves_point_set(point='BLACK_POINT') Set black point or white point for curves
So we paint black or white points at curves. But how? Under what circumstances?
How is Set Curves Point meant to work?