Not so long ago, I had the pleasure of stumbling across damsom plums (I think) at the local farmer's market. So deep blue, they were too special to leave there....
They were a deep blue, like a blueberry, but about the size of a large grape.
A photo to get a sense of the size-

It took a bit of doing to slice these up, but I ended up with lovely tart (based on a recipe from the blog Lucy's Kitchen Notebook). It was tart enough to make your mouth pucker, but vibrant and flavorful, like the crisp air of fall.