Leaves turning red happens in fall, and sometimes spring, from cold weather conditions.
But the leaf you showed looks like it is more of a sudden dryness kind of thing.
I am concerned that the heat you noticed may be killing roots, which can deprive the plant of water (even if you water it, burnt roots can't take up the water into the plant.)
I have killed many blueberries in unsuitable conditions (alkaline soils/water), and from my longest-lasting experimental failures, I would say that cold mulch (bark chips, rotten wood, soil from nearby forests with related plants) and rainwater helps a lot more than anything else.
I think the grass clippings between the roots and the cool soil could be a problem, especially if it's heating up. The grass you are trying to kill under the cardboard could also contribute nitrogen and sugars, causing heat of decomposition.
The heat and root-bound issues could mean you have already lost some roots, and the plants are experiencing transplant shock. The leaves near the crispy one look OK, but smaller and a different color - so the plant may also be trying to adapt to rapid changes in light levels and/or water from where it was raised, to where it was sold to you, to your garden.
If you notice additional leaves turning crispy at the edges, or wilting, you may not have enough roots to support the branches. You might need to consider trimming off some fruiting branches, or at least pinching off fruit, to help the plant survive this experience.
If the branches look very healthy (leaves full and perky, no more than a few odd-sized crispy leaves), you can disregard this suggestion.
If your soil is consistently rain-washed, it may already be acidic, which would be a great advantage. Or if it's got a lot of of decaying limestone, it could be alkaline despite the rain. Do you know your local soil pH?
When I was growing up in an area that does favor blueberries, grass clippings on TOP of the mulch circles around each bush worked great. Helped suppress weeds, trap moisture, slow-release some nutrients.
The plants did not need fertilizer beyond the occasional batch of clippings; however we did have a constant struggle to remove grass and thistles from around the planting area. I would be tempted to dig out a layer of the turf before planting, and put some edging around it, to reduce this problem. Blueberries have shallow roots, and so does grass. I do not have much expectation for grass 'dying eventually,' it is one of the plants most tolerant of cutting and abuse. In my experience, grass sticks rhizomes into your mulch and keeps trying to grow right up into your berry plants.
Blueberries are adapted to nitrogen-poor soils, and to a specific type of symbiotic fungus (Ericoid fungi) that helps them extract nutrients from acidic soils and woody duff (rotting mulch and cool decaying plant matter). I think the addition of soil from a local blueberry patch is a good idea, and may give you other ideas for how to make your home conditions more like the best local conditions for this type of plants. While collecting soil, notice whether the wild berries grow in shade or sun, at top or bottom of slopes, what plants are nearby, etc.
Here is an article I found about other reasons for red leaves - but I don't think this is your main problem in this case.