https://tinyurl.com/American-Furnace

Hi. I stumbled onto this site while researching something else. It seemed fascinating. I did find some info on Google Books that explains the process as set down by American furnace.

I have done a little hobby blacksmithing in the past. We get a nice black finish on iron by using a cotton rag and oil on part in the 500-600 f. range. The burning of the cloth and the oil will impart a nice black color. Not as durable though. The other method is using green coal and heat. So the coal tar thing makes sense.
The use of whale oil I think could be substituted for with peanut oil. The flash point of Flash point of whale oil is around 230 °C (446 °F.
Peanut oil Refined 450°F (232°C

The only thing I think in involved in a heat treat with oil (whale oil was supposedly prized for quenching knife blades also.) Would be it's flash point.

Anyway hope this helps in the mystery. Mike