Cat(h) wrote:
A bit of a mystery, this...
A few evenings ago, I was watering my 15-year old grapefruit, grown
from pip, which lives in a pot sheltered up against the L-shaped front
wall of my house. Having soaked the pot, I sprinkled the foliage, only
to raise a swarm of black flies (as in house flies, but slightly
bigger, just under bluebottle size, and a blacker shade of black).
Why oh why would black flies shelter in my grapefruit foliage? I'd
never had this experience in the previous 15 years!

YURK !! Perhaps it's something to do with your compost. Did you manure?
Eggs where in there and flies hatched and crept up in the foliage?
Something died close by and flies took to the nearest foliage?
Something on the grapefruit foliage, smell/food, attractive to flies?
Whatever, it's horrible !!