Like myself, you may experience that Black Mesa is too dark. The game world slowly fades into an overly dark tone in the span of 3-4 seconds immediately after beginning to play... this is regardless of whatever in-game visual settings you tinker with. The solution is to execute two Steam console commands upon starting the game. These commands solve (1) overall game darkness and (2) lack of flashlight illumination.

1. Set Steam console to activate upon launching the game.
Navigate to Black Mesa within your library. Right-click on the title and left-click on "Properties..." Type "-console" into the launch options text box. Close the properties menu.

2. Launch the game.
Launch Black Mesa and the Steam console will overlay part of the screen. With the console enabled, you can press the tilde "~" key at any time to display or hide the console.

3. Enter console commands.
Type or copy-and-paste the following line into the console and press "Enter."
The first command solves flashlight illumination and the second command solves overall game darkness. The semicolon allows both to be executed on the same line. This must be performed every time you launch the game.

4. Enjoy Black Mesa!
There may be more elegant/automated solutions that exist so feel free to leave comments.