Black point compensation is not working in GIMP 2.10 and also not working in GIMP-2.99
As of git master commit gimp@ebc797a6 and git 2.10 commit gimp@fc5a641d , going to "Edit/Preferences/Color Management/Color Managed Display" and selecting "Use Black Point Compensation" for the Monitor profile is not working UNLESS GIMP is started with the babl transforms disabled.
Of course choosing or not choosing BPC for use with the monitor profile only makes a difference if the monitor profile itself has a non-zero black point. Which is the case for many custom monitor profiles such as are produced by ArgyllCMS for the most commonly used sets of profiling parameters.
Selecting Black Point Compensation when converting to an ICC profile that has a non-zero black point ("Image/Color Management/Convert to Profile") also is not working, leastways not for matrix monitor profiles with point curves that don't go all the way to zero (for which using Black Point Compensation should make a difference), again UNLESS GIMP is started with the babl transforms disabled. It might work with LUT destination profiles such as printer profiles, even with the babl transforms enabled, as babl doesn't handle this sort of profile, but I didn't check.
Black Point Compensation is supposed to actually work. This is the recommended setting for photographic image editing. It does work for other free/libre editing softwares. It works for my CCE version of GIMP. It used to work for default GIMP-2.9/2.10.
My apologies, I don't have the time or energy to regress the code to find out when it stopped working, but I'm guessing the issue is somewhere in the babl ICC profile code.