Just had a thought.
Miss Marple, like Poirot, used her ‘little grey cells’ away of emphasising the importance of using one's intellect and reasoning abilities in solving crimes.
Because she was a lady who was very familiar to the community in which she lived, it enabled her to see or hear things without her presence being remarked on.
IMO, a black Miss Marple in the time these stories are set, would be as unlikely as Alexander McCall Smith setting a white Mma Precious Ramotswe in Botswana.