Yes, there ARE flames deliberately started under the engines before the rocket motors start up, and those rocket motors start off and burn for a few seconds before the clamps release and the rocket begins “liftoff”.
The rocket motors have to start off before the rocket is released to stabilize fuel flow and oxidizer (liquid oxygen) flow to the burners before they can create the thrust to move the rocket up. (Thus the clamps holding everything sown to the concrete foundation.) The turbine exhausts receive the purging LOX liquid and liquid propellant for a few seconds as they cool down, and that mixture is explosive/flammable. If the rocket motor has ignoted, the purged gases and turbo exhaust just pass into the far larger exhaust flames and no problem results.
But before rocket engine ignition, big problems would obviously result. Thus the flames under the rocket before ignition: Those flames burn the vented oxygen and vented propellant at low rates around the bottom of the rocket before they accumulate and blow everything up.
Well, something sent wrong elsewhere, and so the rocket engines never reached “ignition” at countdown steps between 5 and “0” . “0” being “release the hold down clamps”.
Gotcha and understand.
Sort of like burning off vented flammable gas from anything... refinery, hydrogen reduction furnace, etc.
So the black sooty smoke from something else burning is most significant.
Not at all the same... but I can’t help but remember the puff of black sooty smoke at the Challenger liftoff.