Wild Blueberries are fussy about where they grow. You already have pretty good conditions for them or they wouldn't be growing there now. However, one thing you can do is make sure they get enough sunlight. Too much shade can eventually kill them.
I have a patch of lowbush blueberries that came up after I logged. All I have done is cut back a few trees around them and they are thriving.
I have cut out what trees that were in them and will eventually will open the canopy more around them. Was looking for any fertilizer recommondations or possibly lime?