I thought my blueberry bed had acidic soil... nope, I bought a pH meter, and it's nearly neutral. No wonder the bushes are a yellow green and I get a couple handfuls of berries, at most. I have added coffee grounds, oak leaves, peat moss, pine needles. I give them the whey when I make yogurt. This year, I bought a soil acidifier amendment, but it can take a year to do much.
I'm really tempted to try the vinegar idea. 1/2 tablespoon cider vinegar in a gallon of water is what I just read online.
The coffee grounds are from Starbucks. Their label says they are not acidic, coming in at the 6.8ish pH range. I took a scoop of grounds, mixed them with water, and checked with my pH meter. Yup, just barely under 7. I used filtered water that came tested at 7.