Glad things are straightened out for you.
Im not so sure of your hypothesis but you are on the right track. Devoloping the proper coal bed is paramount. Yes it is good practice to use coals for heat/lighting splits and splits for smoke/creating more coals. If your running a fire thats too big too clean it wont be black just your food will have a very light smoke. The trouble comes when that fire dies and the next split doesn't ignite cleanly.
I guess that has been my problem generally when cooking low-and-slow with wood... my fires have always been way too clean, so I've gotten very little, or no smoke flavor. Also, little bark. I got good at building fires this way, and led to good food, but the lack of smoke flavor always bothered me. I would say that on all of those cooks, I've always had a bright yellow flame, just a little blue smoke, or none at all. If I had to estimate, I think my ratio of coal bed to fresh split (by volume) was about 1 to1 during those cooks.
The black smoke situation has happened twice to me, both times when I was trying to cook much hotter than usual while using the same fire technique ( 1:1 coal bed to fresh split ratio), So when trying to keep a temp around 325 to 350, that just left me with a furious yellow-flamed hot fire (with the door closed and vents wide open). I'm thinking that burning wood with that much of a bright yellow flame leads to bad cooking smoke. Very thin black smoke, but black nonetheless.
By contrast, during my last successful hot and fast cook, I maintained a ratio of around 3 to 1 coal bed/fresh split. This heat from the coals allowed me to choke back intake, while maintaining the split well lit , but with a relaxed slow rolling orange-bluish flame. If the coal bed wasn't there, the split would have smouldered too much with the amount of air I was giving it.. But as it was, it seemed to strike the right balance.
I'm looking forward to my next cook to see if I can repeat the experiment so I can report back!
