It could be a number of things Gavin, such as over fertilising, soil not acidic enough, heat stress, not enough water or too much water, a fungal disease, or even natural dieback as the plants move into the cooler season.
I have seen the same thing in our blueberries and a little more regular water seems to improve this disorder but I would recommend going through a process of elimination starting with what you think might be the likely cause and correcting that to see if it improves the situation.
Yes, what Mark said!
I have several blueberries that I recently repotted into 12lt white buckets which will be their permanent home for a few years.
I made new potting mix and added extra fertilizer to get them going before winter hits as most of mine are warn climate types. I have put them in a nice sunny sheltered position.
Yesterday I saw the new growth on two have red and white patches much like yours.
I put it down it incorrect pH due to over-fertilizing or over watering.
Because it must have occurred as the leaves were forming which was right after I repotted them.
It could be due to the new potting mix holding more moisture than the old mix did which has rinsed out the soluble elements such as nitrogen but not the heavier trace elements so now the mix is unbalanced.
I will just keep an eye on mine to assess the next set of leaves that grow and see if there is an improvement.
If leaves die off due to old age and cold weather, they go ruby red then maroon and always from the bottom of the plant upwards.
Hi, yeah I concur with what others have said, leaves turing red can be caused by a quick cold snap but won't be evident until a week or so later, soil ph causing a lockout of phosphorus and or just a lack of phosphorus. If you haven't been feeding then unlikely a ph issue, go for a berry fert higher in P#
By the looks of that photo, I'd say it's either not getting enough water, or it's fertiliser burn. I had the exact same thing happen on my bushes back in January where I wasn't able to water them for one day, except it wasn't just the leaves that did that, but some of the canes as well. Blueberry plants are water hogs and I'd incline for them to be watered daily especially if they're in containers; I've never experienced or heard of a Blueberry bush get problems from too much water, but they do not react well at all if they're not getting enough.
There's the possibility of too much fertiliser as well. It's quite an easy thing to do if you're keeping them in containers or pots as you're doing. There's also an outside chance it could be the soil PH being outside the prescribed range (4.7 - 5.5 is fairly safe) so it might be worth testing the acidity.