Hey there. Thanks for your report. Do you have HDR enabled?
HDR Brightness/Darkness is based on your SDR settings. If you have HDR enabled, following these steps should improve your experience:
1. Quit D2
2. Turn off HDR in console settings
3. Start D2. In Settings, set the Brightness to "4"
4. Quit D2
5. Turn on HDR in console settings
6. Start D2. In Settings, follow the instructions to set the HDR White Point to taste.
7. Then set the HDR Black Point in the exact middle (49-50 clicks on either side).