This is what The Dehydrator Bible by Jennifer MacKenzie, Jay Nutt and Don Mercer have to say about it:
You use a process called “checking”.Checking is a technique applied to berries with a waxy coating on their outer skins, such as grapes and blueberries. In the checking process, you are trying to create small disruptions in the waxy layer. These disruptions will appear as small cracks and may have a checkered appearance. Without these small breaks in their skin, many berries will not easily give up their moisture in the drying process.
To “check” berries, place a small batch in a heatproof strainer and dip them into a pot of boiling water for the recommended time. You must be careful not to heat them long enough to split the skins if you want the berries to maintain their shape and appearance. With a little trial and error, you will be able to determine the proper checking time for particular berries. The times given in this book are meant as guidelines only and are for representative berry samples. Your times may vary depending on the size and nature of the berries you are using.
Linda's Note: It's kind of like blanching. Just keep them immersed until the skin cracks, then immediately plunge them into cold water.
If you find that checking does not work well for you, you may pierce the outer skin of each berry numerous times with a toothpick or the tines of a fork. This approach is particularly effective with grapes, which are large enough to easily pierce with a fork. We did notice some flavor loss in berries that were checked compared with those that were pierced with a toothpick. However, piercing individual berries is time-consuming, so you may decide that the slight flavor loss caused by checking is a minor sacrifice.
Linda's Note: With grapes, I found the easiest thing to do is simply cut them in half.
LINDA’S NOTE: When I looked up dehydrating blueberries, I could find nothing about a specific length of time to do the “checking” process. Just watch them carefully and see what happens.