What about cranberries? They grow in acidic soil.

I going to try wild huckleberries under my rabbiteyes. Both have the same soil requirements but the huckleberries prefer some shade.

dt

strawberries also are a good pairing with blueberries

I grow Garden sorrel under my blueberries, they like acid also. I like the contract between the different color greens and it is a perrenial in my zone. I was going to grow cranberries or ligonberries but I read that they are shallow-rooted like the blueberries so i thought that there would be too much water and surface area competition. Look for something with deep roots like Sorrel or Strawberries.

Contact the horticulturists at Raintree Nursery (in Washington state but they're online too) and they'll have good ideas that are safe. I would have recommended cranberries too, because of the acidity... but I don't know what works with the shallow root system of blueberries. They've "got everything" and are very nice.

I read in an European gardening book that heathers make a good companion plant for blueberries. They do have the same/similar requirements and I like that the REAL heather is useful for tea and as medicine.

Here is a link that might be useful: Heather specialists