If those blueberries are from Michigan they must be inside. I think that is a potassium deficiency. It could also be fertilizer burn from too much fertilizer which have burned the roots, or drought stress. Here is a link to some information of potassium deficiency. https://www.canr.msu.edu/uploads/files/Potassium%20deficiency.pdf
I generally recommend fertilizing potassium sulfate not, potassium chloride the common potash fertilizer. For small plantings I recommend using a rhododendron/Azalea mix such as Mir-Acid which has the correct blend of nutrients.