Here are one or two possibilities. Blueberry scorch or shock viruses. When you are able to answer the questions I've asked in my first comment we'll be able to ID more closely. This link I've sent looks like the pictures of your blueberry and by what you've told us might just be correct.
You said you used ericaceous soil for that planter. I am assuming you got soil from your garden where blueberries or ericaceous plants were grown. That just might have been the problem. Virus, even other disease vectors would be present in that soil. Using garden soil in pots and that includes planters is just a bad idea. In the large body of garden soil we've no idea what is in the soil dug up to put in the pot or planter. In the large body of garden soil are 'controls' that will never be found in that soil after removed to be placed in a pot.
Lots of plants, if they are healthy because they are fertilized properly, watered properly, have great drainage would be able to fight some diseases. But yours sound as if they suffered some major stress and might have been made more susceptible to disease as I've said earlier.
Your reaction was actually an excellent reaction. To get rid of the soil and wash the roots. If you had used sterilized potting soil in the original planting (usually on the acidic side any way) that would have made a difference. But a virus is tiny tiny tiny and washing the roots wouldn't have helped because your plant would already have been infected. Fungus is the same way, there are a few blueberry blights that could also be culpable.
I don't think your shrubs will make it but no harm in trying. blue berry scorch virus Check out these pictures. I am amazed that all these pictures look like any and all diseases lumped together! Then there is 'shock' and that again looks like your blueberries. shock and scorch There are preventative measures which would mean doing something before infection. Shoot. Wait to hear from Bamboo. Please send more pictures; including standing back to give us a picture of the planter and shrubs. Tell us more about the soil you've used, especially the first planting. What was grown, why you believe it is 'ericaceous' soil and what you've added; what kind of compost, fertilizer. Did you install gravel or rock at the bottom of the planter before putting soil in the planter (not good for drainage)? You do have drainage holes, correct?
If you've got great drainage in those planters water will not be a factor. Blueberries are essentially bog plants and love low pH and lots of water. With what little I know right now, I would be getting rid of those plants, the soil, bleaching that planter and planning to replant with plants that are not in the ericaceous family. Wait for another answer however, okay? Have you grown blueberries before in your garden or is this a newly adopted garden soil from a previous owner?