Several points:
a. Many steam locomotives had 'netting' in their smokeboxes meant to trap large hot clinkers and sparking bits of carbon, unburnt material, and such. The netting was attached at its upper end atop the flue sheet and was anchored at the lower forward end of the end-cap that is the smoke box cover with door. Hot gasses and crud was emitted from all the flues and went up the stack from there.
When the netting got too fouled, it greatly affected the 'steaming' of the locomotive. I read one story where an engineer had to ask his fireman to take his seat while he went forward along the running board, slid back the dogs, opened the smoke box door, and did something to free the netting from what he new to be impairing his locomotive's performance. When he returned, the steam pressure gauge showed more pressure soon and the locomotive was able to speed up.
b. Supervisors would cary smoke density cards with them. As they drove near their rights of way, tagging along revenue consists, they'd hold up the card and look for a match with the apparent smoke density emitting from the stack. If the smoke was too dark, that hogger would be written up for not correcting his fireman sooner. Dark smoke was a sign, not of an engine hauling harder, but of over-firing for the current demand on the boiler and reverser setting. IOW, wasted fuel blowing out the stack and settling on your mom's freshly washed and hung sheets near the tracks. And a lot of mom's did that....living near the tracks and hanging sheets.
c. Sand is routinely used to clear the flues if the fireman or hogger suspects they are getting carbon/soot stenosis. The fireman uses a coal shovel to dip into a pile of sand and he feeds it into the clamshell firbox door by letting the intake through the open door draw sand grains across the fire and into the flues. If that procedure were underway, yes, you'd get darker smoke as you might expect, and there would be no blame on any of the crew since that was legitimate maintenance. Just not near mom's sheets....thanks very much.