Short for black lead, n., graphite, or plumbago. Only with reference to its use as a material for pencils. Hence, a small stick of graphite for…

a piece of pencil lead of a suitable length for a pencil.

graphite (formerly blacklead) as used for making pencils; = lead, n.1 3; a slender rod of this used in a wooden pencil or a propelling pencil.

That which serves to refill something; a fresh supply (esp. prepared or packaged) of the material with which a device must be loaded.