You asked, “What in hell has the color of the smoke got to do with it?” Son, you know that a fire is not as hot if it is oxygen starved, which is why it belches darker smoke. You really should stop before you remove all your covers.
No, black smoke can also be affected by WHAT is burning. Oil smoke is usually black. Some chemicals burn with black smoke. What was coming from the WTC was both smoke and DUST, mostly dust and particulate matter. That which fell down was the dust and particulate matter. That which rose was smoke along with finer dust and particulates. There were a lot of synthetics in the building that would burn with a blacker smoke. Your claim that there was no compression is entirely wrong.
I am well aware of oxygen starvation in fires. . . but are you aware of the reports of the strong street level winds TOWARD the towers. They were drawing air toward them. There was a roll of cloud from the ground up, then back down.
Or if combustion of the microparticles in the smoke is essentially complete, they will stop radiating visible light...