Add half a container of blueberries when you add your bag of cranberries to the sauce pan.
(I guess that's half a pint of blueberries to a 12oz bag of cranberries, unless you get the organic ones, which are only 8oz a bag, and then you have to use two of those for enough sauce, so I guess half a container of blueberries to 2 bags is OK then too.)
And don't forget to use orange juice instead of water! The recipe on the back of the bag can't tell you what to do.