Here's an interesting article from the UK's Telegraph about a very rare variety of plum, or "Damson", called "Blue Violet":
An amateur horticulturalist is nurturing what are believed to be the last wild-growing examples of a rare damson tree.
The Blue Violet has long been a rarity, even amid the celebrated orchards of the Lyth Valley, Cumbria.
But in the last century their numbers have declined to what experts suspect are a mere five - two of them in official "museum" orchards, and one other clinging to life in a pot.
The Blue Violet are apparently bigger, juicier, prettier, and harvest much more easily than other varieties of Damson, and the horticultural world is very excited by the find.
Damsons originated near Damascus, and were introduced to England by the Romans.