Leaves turning yellow is a phenomenon called chlorosis. At the most fundamental level it is a lack of chlorophyll (the green part of the leaf) so you see the yellow pigment.

There are many reasons for why this would happen but it is a symptom of stress. In Bluberries a leading cause of chlorosis is iron deficiency.

This is from Kansas State
"Affected leaves turn a yellowish color while the leaf veins remain a dark green. Iron chlorosis is caused by the plant not being able to obtain the iron it needs. Iron is needed for the
production of chlorophyll and therefore, a lack of iron results in a loss of the green color in the leaves. In severe cases, leaf color may change from yellow to white to brown. Blueberries are especially susceptible because they require an acid pH (4.8 – 5.2 is best)."
http://www.hfrr.ksu.edu/doc3456.ashx

You should do a soil test to rule this out.

Since your plants are in pots it is possible that the soil has become depleted. Re-potting them would allow you to provide fresh nutrients and examine the roots. Is the plant root bound? (i..e growing up against the side?)