1) In lead-mining this was waste material and it occurs first in an account of slag that was being reworked.
1427 antiquum slagwerk ... blakwerk a bolisted usque smelting place, Weardale
1527 nor also of the blake warke, Bishopside
1537 saving to me & myn executors the oore leade and black warke at the same, Derbyshire. In the fur trade ‘blackwerk’ was the skin of squirrels taken in winter from Central Europe, where the back is dark grey, almost black.