Step 1: Get a texture that has a gradient fade from any color ( it doesn’t matter what color, you’ll see why ) to fully transparent. ( This is supplied in L4D2 Tools, I don’t know about regular Source SDK though. )
Step 2: Place it in a circular style around your sky_camera in the 3D skybox, like so;
Step 3: Match your environment fog with your sky camera fog ( distance, far z clip plane, colors, etc. )
That’s it, you can use any colors, clip plane, skybox, whatever. Don’t have a texture like the one I described? Oh well, sorry, make one or find another way.
This is the end result;
As you can see, the fog blends into the sky ( right portion of the picture ) meaning you can use any skybox you want. It also creates a silhouette around anything in the sky, so…
Pick one: Skybox fog that blends with your world fog and the sky or 3D skybox entities that maintain their visibility.
So yeah, this is how you blend your fog with your skybox. I’ve explained this many times in threads like these, it should be stickied or some shit. Until then, I hope this helps.