This wild European plum is the fruit of the blackthorn (Prunus spinosa), which also bears showy white flowers. The purple-skinned sloe has an extremely tart yellow flesh. Though too sour for out-of-hand eating, sloes are used for jams, jellies and to flavor liqueurs such as sloe gin. They're not generally available fresh in the United States.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.