Since no one else has answered, I'll try to help. I usually grow blueberries in soil, but I also grow lots of vegetables in containers, just not blueberries. So take my advice for what it's worth.

It doesn't appear to be over fertilization, as that would affect the entire plant, but much of it looks ok. It doesn't look like pH either. From the photos it appears to be specific branches that are affected. You could check for something that's affecting entire branches such as borers, fungi and such but blueberries are usually fairly pest resistant. Considering your zone though, might be worth checking for pests or diseases that damage stems. Aluminum can be toxic at high doses (I usually use sulphur), but I don't think that's it either. If you do think you over fertilized your plant, flush it.

Another possibility is root damage. I have noticed that root damage on blueberries often only affects a few canes at a time, almost like canes each have their own root system. Root damage can be caused by many things including overwatering, rot, pests, or even physical damage among other things. Blueberries definitely don't like wet feet. Maybe even frost damage since you are in a pot rather then ground. Might want to pull the plant out of the pot and take a look. Looks like you may have other plants growing in the pots. Could they be choking roots?

Let us know what you find out. It's hard to tell what is going on without more info.

Thanks for your response. I checked the leaves for bugs or fungus, and I couldn't find anything that I would say is the source of it. However, since this has affected my plant over the past two weeks and it isn't getting any better, I decided to repot.

I examined the roots and they appear to be healthy. I'm a novice gardener so I may have missed something. I cleaned the roots pretty thoroughly. I did find a couple of earthworms within the root ball. Don't know how on earth they got in there. Amazing creatures.

Anyway, mixed up a batch of 5-1-1 minus the lime (because it neutralizes the PH and I'm already using a fertilizer with calcium), ... and then repotted.

I also trimmed back the dead branches.

This may be a little radical since the plant was not dormant (best time to repot as I understand), but if I did nothing it looks like the plant was going to die. Just hope I did the right thing.
Thanks again for your comments.

They should be happy in 5-1-1 without lime. Make sure also that your fertilizer has magnesium. If not, you'll need to add epsom salts. Blueberries are pretty hardy overall especially when the pH is right. You are doing all the right things without knowing exactly what happened. It may have been as simple as winter kill. They usually don't require much feeding though. In a raised bed in soil, I only have to add ammonium sulphate, epsom salts and sulphur to keep them happy. In containers you will need to add other elements, but just keep in mind that they don't need a lot of fertilizer. Good luck!

Good Point. I'll check on that magnesium, but I believe I have it covered. I use foliage pro at half dosage as well as Earth Juice. Thank you for your comment.

Stop using Foliage Pro on your Blueberries. Blueberry plants do not like Nitrogen in a nitrate form. I can guarantee thats your problem using Foliage Pro.

You would think that all the Blueberry plants would show the same signs, weak growth, dying leaves, etc. since you have used the same watering and fertilizing on all the Blueberries. I do see signs on your other Blueberry just not as extreme. I would say one is a lot healthier than the other or the one received too much Foliage Pro.

I use Seaweed Extract and or Neptunes but thats just me. I'm sure others will respond to more fertilizing options so the choice will be yours.

Ron