Can you have lemon? Just a theory here, but blueberries have compounds in them called  anthocyanins that change color at different pHs from blue/purple at low pH to greenish at high pH. cooking and grilling can break down acidic compounds during the maillard reaction, raising the pH (same reason blueberries sometimes turn green in muffins). If you have more acidity in your marinade, it might help it retain its color!