Cel shading in BM?

Cell shading would be a cool novelty I guess. No point in it really though. For starters, that kind of visual style wouldn’t suit a game like Half-Life/Black Mesa. It’s meant to look and feel realistic.
Nice thought though.

look? maybe. Feel?

Nah.

Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.