Quite frankly this colour has a long and dubious history, establishing exactly which colour relates to the name is very difficult and particularly which pigment is the most authentic and lightfast and then this is still probably a long way from what the man himself used!
However I believe it has continued to be produced for so long because it has a very specific and useful purpose on the palette.
To the paint...
A very dark, mid-temperature, black/brown. Super smooth and buttery in handling.
Very useful to neutralize mixes and form chromatic greys, as once white is added creates almost neutral greys.
Pigment - PBr8
Vehicle - Linseed Oil
Opacity - Semi-Transparent
Tint Power - Average