Pine bark fines and spaghnum peat moss 50:50 works good. The peat moss will work by itself.

if you want the easy way out...most garden centers sell potting mixes for acid loving plants (azealas, etc.). This has worked well for my sister.

I use the DWN suggested mix. It's worked well for me for a few years now, and I've been bringing in lots of berries since early April.

I have used 1/3 peat, 1/3 potting soil, and 1/3 pine bark mulch (fine). Also a little sprinkle with Hollytone for acid loving plants. I do the occasional coffee grounds or watering with 1/3 coffee and 2/3's water and mine look great. Have some in pots and some out in the garden.