I can only guess but blueberries have very shallow root system and it looks like you may not be watering enough. You should irrigate often but not for long. Only wet the upper soil profile where the roots are plentiful. Too much water and you will develop root rot. Key words are frequent but short waterings. Good luck.

Thanks very much for your response. Though I agree it looks like a lack of water, I don't think it is a watering problem, as I do water as you describe. Also, it has seemed to start at one end of a row of blueberries, and progress through the row as if it is contagious. I know it is hard to diagnose with just a few photos and not much information, so thanks for your efforts!

The symptoms shown in the images are related to sun/heat and water. The leaves are just not getting enough water to keep up with the heat demand. This can be due to many things.
Not enough water on your part is one. I jumped to that based on the poor grass growth in the background of one of the images. Maybe you are using a drip or drip-like system. We often have growers use two drip lines on opposite sides of the bushes to get enough water to them. But you say you are providing adequate water.
The other problem can be lack of roots. The most common cause of that is a root rot due to overwatering. A water mold called Phytophthora is very common and you can find out about that here:
http://pnwhandbooks.org/plantdisease/blueberry-vaccinium-corymbosum-root-rot
There can also be odds and ends of canker fungi that can interrupt the flow of water to the leaves. A close inspection of your root crown area and long the trunk, branches and stems will find those problems.
Finally, the most common problem with blueberries is soil pH. The soil pH should be between 4.5 and 5.5 so what is the pH of your soil?