It's a curious thing, but plums with red or purple-blue skins always get tarter and livelier when they are cooked. In fact, the bluish-purple prune plums (the oval-shaped fruit also called French or Italian) are downright bland raw and, to my taste, good only when cooked. As to yellow and green plums, I can't vouch for their behavior because they never seem to capture my attention in such vivid company.
Tartness gives cooked plums versatility. When we expect a houseful of people, as when friends visit our Provence retreat in September, I keep a big bowl of plum relish in the refrigerator. It's a lively accent for simply cooked meat or poultry, splendid as a condiment for curry, and intriguing in a ham sandwich. It's even quite nice with yogurt for breakfast.