Rotten oak leaves. I add more leaves to the surface each year. I've tossed down some compost, then mulched with leaves. The oak leaves have a pH of around 5.6, so the plants love it. The moisture remains long after it rains, and my worm population is thriving. Depth of mulch is about 4 inches.
Last year I had troubles until I got more fence up. The bed was doing well with the compost and mulch, lots of bugs and grubs. Couldn't keep the chickens away from it. I had 6 blueberries, they scratched up 5 of them.
Seed the Mind, Harvest Ideas.
Just don't fall into the trap of using coffee grounds for blueberries, or any other acid loving plant. Coffee grounds lose their acidity once they are brewed. Unbrewed, they are highly acidic, but once brewed, their pH raises to +/- 6.9