There are several amino acids in normal sweat and some may promote reactions with some sulphur compounds (
disulphides*), also present in sweat, and the
structural protiens found in fingernails. Mild sulpheric acid can react with steel to form a black iron oxide (Fe3 O4) . This black oxide is actually a protectorant layer and will inhibit that red rust you normally get. I'd leave this black oxide alone...maybe a wipe with light machine oil.
Henry Taylor tools come with a blackened shank...not painted, but chemically blackened to prevent rusting.
*The sulfur atom binds readily to
heavy metal ions. Under oxidizing conditions, two cysteines can join together in a
disulfide bond to form the amino acid
cystine. When cystines are part of a protein,
insulin for example, this stabilises
tertiary structure and makes the protein more resistant to
denaturation;
disulphide bridges are therefore common in proteins that have to function in harsh environments including digestive enzymes (e.g.,
pepsin and
chymotrypsin) and
structural proteins (e.g.,
keratin). Disulphides are also found in peptides too small to hold a stable shape on their own (eg.
insulin).
And ya, I had to look this up, but I seemed to remember something from dealing with tool steels about coloring from different acids. If we've got a metalurgist or chemist on board, maybe they can give a better explanation.....I won't claim to be either, and this is just my best guess.
Al