We always called those dark blue oval shaped ones Prune Plums.

I am growing Stark Bros. Earliblue to use to make prunes. Last year was the first year I got a crop, it was a very small crop, so I ate them fresh. This year the tree is loaded and should be ripe about July 15th-20th. At that time I will be cutting them in half and drying them in my dehydrators, I have a Nesco and an Excalibur (the Excalibur was Jellyman's recommendation in an old post) You can pop the backs or as some instructions call it, turn them inside out for quicker drying. I have dried other fruit and have done it both ways. It is easier to just half them, place skin side down on the trays and wait a little longer for them to dry. I store all my dried fruit in jars or plastic bags in the refrigerator so I am starting to leave a little more moisture in the fruit.

There is your method, hopefully someone else can comment on how well these or other prune/plums do drying, otherwise post again at the end of July and I'll let you know how mine turned out.

The best prune tree to grow is the one that will thrive and produce in your area. South of Virginia, plums and prunes can be difficult to grow, but special research programs have developed trees that are more disease resistant. You might want to do a search of this forum for prunes or plums and Virginia.
LongJohn prune from the New York State fruit testing program is supposed to be vigorous and delicious..
Here in SoCal, my young Sugar prune has set around eight to ten prunes its first year here. One split, but the others are developing nicely. Those that aren't eaten fresh will be dried in my Nesco dehydrator.
A fruit enthusiast in Northern California said his favorite prune was the Moyer. In France, the Prune d'Agen and St Catherine Plum are favorites. The California French Petite Improved is supposed to be an improved variety of the Prune d"Agen, but IÂve tasted both, and like the Prune dÂAgen better, although both are very good.The French somehow slightly rehydrate the Prune d'Agen prunes, so they are soft, chewy, and more moist than the French Petite Improved that's sold in the grocery stores here.

Thanks for the replies, right now I have 2 plum trees (Shiro & Yellow Egg) but both are no doubt too juicy to fill the bill.

Hoseman, yes please do let me/us know your results this season as I have bought Stark trees in the past with good success and that Earliblue sounds interesting if it's well suited for my zone here in Va. I assume that Earliblu is somehow related to Damson? I don't need any fancy variety just one that's easy to work with and lasts in storage (either room temp or fridge). Store bought prunes are probably treated with a preservative to hold at room temps though.