I'm assuming it is carbon, but it might not be. What causes the black (carbon?) in intake runners and in a carb?

Ran a 750 Holley vacuum secondary carb maybe 10 to 20 hours last year on my daughter's 67 coronet with a 383. Was freshly rebuilt when I installed it. The carb is now heavy black.

My cousin's 400 cid 67 Firebird with a Paxton supercharger has half the runners black and half clean. Its a dual plane that we now know is a mistake. The cylinders that are rich have the black runners.

What is causing this?