Are you watering the leaves? Don't.

Has it been really hot? Give them some shade.

I kind of doubt that the azalea mix and compost will give you a low enough pH for blueberries--or drain well enough for a container planting. I recommend taking a look at Dave Wilson Nursery's suggestions for growing blueberries in containers. I've followed their growing mix recipe and it has worked well for me.

Here is a link that might be useful: Dave Wilson Nursery Container Blueberry Growing

Yes, having the correct potting mix (pH) is key. You might have to repot them, and remove the bad leaves. Then you should be fine. I have several BB bushes in containers, and they are potted in pine bark mulch and peat....so far they are growing like weeds. Also when you water you might have to adjust your water pH with vinegar or something. My tap water pH is around 8, so I add about 2 tablespoons of white vinegar per gallon of tap water.

All good info so far. I would also add that it wouldnt hurt to prune out some of those really thin branches and shoots. On young plants like that, I like to remove the really thin branches/shoots and leave only the the thicker ones. It makes watering easier, opens up the plant for air circulation and sun, lessons the chance of getting water on the leafs if watering during the day. If you do prune them be sure to sterilize your clippers with a bleach rag between every cut. The plants will respond very well to a good hair cut!

For potting mix I use 70% pine bark mulch(not nuggets), 20% peat moss, 10% perilite and about 3 table spoons of sulfer(Im using 15 gallon pots). That with some good organic fert has worked very well so far for me. Last but not least, lots of rain water! You will run into problems using municiple tap water sooner rather than later. If you must use tap water you can either acidify it (I use sulfuric acid when I have to use tap water) or use Epsom Salts to counteract high ph tap water. Ive never used Epsom Salt but have read that it will work. Good luck.

I can't thank you all enough. I'll give the peat moss and pine bark mulch thing a shot. I'll also add some white vinegar to the water and make sure not to get any on the leaves. I'll keep you all posted. Fingers crossed.