Actually it is the flag of St. Piran, the patron saint of tinners but it has been adopted (unofficially)
as the Cornish National flag as St. Piran has been adopted as the Patron Saint of Cornwall.
Legend: St. Piran "rediscovered" tin-smelting (the Romans had smelted tin in Cornwall, but the methods had since been lost) when his black hearthstone, which was evidently a slab of tin-bearing ore, had the tin smelted out of it & rose to the top in the form of a white cross (thus the white cross on a black ground on the flag).