I am now sure that "acciaio nero" is best translated as "black steel".
I found out that
"acciaio nero" is primarily meant to be "acciaio ricotto" (annealed).
However it might also mean covered with carbonaceous materials (black graphite) or painted black, depending on context.
Further, black steel (annealed steel) need not necessarily be carbon steel. Annealing, (a heating process carried out in several different ways in order to reduce internal stresses of the material) can be carried out on many different kind of steels, and on other materials as well.
HTH,
F