Bill Reynolds wrote:
I have a small patch (3"x3" ) of a black mould on a piece of drywall.

What is a good way to kill it?

TIA

Spray with bleach solution, wipe down with soap & water, & paint over
it.

Damp mold is easy to kill. Dry mold gets airborne real easy when you
touch it. Painting over it seals in into the drywall and keeps it from
spreading or getting airborne.
And contrary to other's opinions, there are literally 100's of
different types black mold and only 3 are dangerous. These 3,
Stachybottrus, Aspergillis, & Penicillium can have serious health
effects. The rest are relatively harmless.

Bob S.