I had this pretty common issue(1 2 3 4).
FIX
1-Right-click on black mesa title under the Library in Steam and select Properties.
2-Click on set launch options
3-Write -w -h in the text field,
where and are respectively your screen width and height.
4-Run Black Mesa and have fun!
If you don’t know your res, you can find it in control panel->Screen or checking in your Gpu software.