There are some potential short term fixes for this, until we fix it in a patch.
The game is fullbright and it could be for a few reasons.
Find ‘autoexec’.cfg in SteamApps/common/Black Mesa/bms/cfg and open this file with Notepad
Add the following lines to this file:
mat_dxlevel 95
mat_fullbright 0
mat_hdr_level 2
Save the file and launch the game to see if this fixes your issue.
If you have a graphics card, it is possible your hardware drivers are forcing bms.exe to use the integrated graphics rather than the graphics card. Check in your driver program to make sure bms.exe is set to use the graphics card and not integrated graphics.