Thank you for your question.  This type of reddening of the veins is typically caused by one of two issues.  The first is a nutritional deficiency, which cannot be tested without having a soil test for both soil pH and nutrient values.  Soils must be low pH in order for blueberries to take up critical nutrients.  It would be helpful to know if this coloration is apparent on only old foliage, only new foliage, or both.  If you have not had your soil tested, I suggest you do that.

The second cause is the plant's absorption of an herbicide.  I don't know what, if any, herbicides you use, or whether the affected plants occur along a property line of another farmer whose herbicides may be drifting to your plants.

You might want to take the leaves into a local plant pathology lab, along with getting the soil test, to narrow down the issues.  Good luck!

You might find this helpful:  http://extension.missouri.edu/blueberry/documents/Shared_Documents/MOBBSchool/MOBBSchoolConf11/Nutri...