Hello Exile!
The black stuff most likely was soot - very fine soot was frequently used for its mild abrasive power, chiefly for imparting a shine to the blade. Candle-snuff and tallow was supposed to make a good dressing. Chimney soot has often been used too.
Be careful of the cracking on old strops - no amount of dressing, lathering or compound will get rid of a deep crack - it's dead leather, it can't heal! :) Most likely the cracks were just surface crazing.
Regards,
Neil