At the start-is that the equivalent of running "rich"?
With the black smoke?
More or less.
Firing a coal fire is a skill, and usually black smoke gets assigned as "wasting coal" or "inefficient combustion."
My GUESS in this case, and the N&W engineers and firemen did nothing but run this grade all day in these locomotives, is that you're seeing the result of the fireman REALLY loading up the fire before the start of the grade, and it consequently cooling off and not burning as efficiently. The N&W ran coal from the mines they served, which was primarily fairly high grade bituminous coal. Bituminous basically burns in two stages-the coal initially "cokes", driving off the volatiles that burn in front of/in the flues, and then the remaining carbon gives a long lasting stable fire.
Adding coal always disrupts the process a bit, so on flat land at steady speed the fireman would likely add small amounts of coal continuously(using the mechanical stoker-there's no way even 2 or 3 people could keep up with hand firing one of these beasts).
Climbing a grade that steep with that much tonnage in tow is going to use a LOT of steam, and you need a stout fire to keep up with the steam demands, plus that much steam is going to translate into a lot of draft that will help the fire recover quicker and also risks pulling the fire up off the grates if it's too small. Loading up the fire ahead of the grade would probably be smokey at the start, but you also have 8 miles ahead of you to drag out at ~20mph, and you really don't want to miscalculate, find yourself with too small of a fire halfway up a grade, and then have to over-correct and maybe find yourself not even able to make enough steam to keep moving before you get to the top.
I'll also mention that now, when you see a steam locomotive running, people expect black smoke. They usually will intentionally make it for photo run-bys and the like, although hopefully won't run the whole trip that way.
(on oil-fired steam, even if the mix is less than optimum, the fire is rarely sooty in the way a coal fire is. Oil does need to have the flues periodically cleaned, and that's done by quite literally dumping a bunch of sand into the firebox and letting the draft suck it through the flues. "Sanding the flues" does give a great display of thick, black smoke, so many oil operators will choose run-bys as an ideal time to sand).
