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."
http://www.davewilson.com/br40/br40_trees/plumPrune.html
Maybe you'll recognize the variety from the various pics of the varietal fruit. Hope this info helps.
Regards, GF.