Bob H writes

We don't have any nearby plants in the house, and I couldn't see
anything moving around on the leaves or the ontop of the soil.
The black soot is only on the underside of the leaves now I have taken
more notice!

Any ideas as to what a cure might be?

If you can't see any insects (they'd be staying still and chomping, not
moving), look on the underneath of the leaves for round brown scales,
especially along the stems and leaf veins, which can be removed with a
fingernail. If you find these, it's scale insect, and you need to deal
with these to avoid a recurrence of the black stuff.

The black stuff can then just be wiped off.
--
Kay