I ask for help on behalf of my mother, who has a potted blueberry plant in her garden: the label says Vaccinium corymbosum, "giant American blueberry". About a month ago my parents left for a week, and since no one was there to water the plants it completely dried out. After a while it started sprouting again, but never completely recovered: branches have dark/grey marks and leaves soon become red, then develop black spots and fall out. She tried repotting it but without any effect. The blueberry has been watered daily and is exposed to full light during the course of the day. Summer has been quite hot here (almost everyday around 30 C°) but I don't know if that could be a factor. (I'll load pictures as soon as the stack exchange app allows me)
Intense heat and sun for blueberries should not be a problem as long as the transition is not sudden, that is moving from inside to outside in a short time frame.
With blueberries the thing to watch is the acidity of the soil it is growing in. They require acid conditions and not providing this can play havoc with the health of the plant. There have been previous discussions: see for example "Is it really important to give blueberry plants acidic soil?" This is the primary consideration with this plant; if you have been careful with this then please let us know.