Black lead is indeed graphite based.  The substance is used for
blacking and polishing cast iron grates and kitchen ranges (fireplaces)
and was sold in the UK under the trade names of Zeebo and Zeebrite.

During my lifetime as a leaded window maker I used it for blacking and
polishing the leadwork on completion of a job.  I suggest that it would
be sold by anyone who supplies the stained glass trade.  Search for
'Black Grate Polish'.

Keith Reedman
Long Eaton, Derbyshire, England