Were they a sort of dusty blue? Did the stone come free of the fruit easily? If so it was a freestone variety. Most likely either Italian prune plums, or Stanley plums, as those are the most common in US stores, though it could also be a "Sugar Prune". Have a look at the link, scroll all the way to the bottom to see "Sugar Prune."
Maybe you'll recognize the variety from the various pics of the varietal fruit. Hope this info helps.