Blueberry yogurt in place of the sour cream, and blueberry juice/ nectar (you may need to find an organic store) in place of the water. I've only tried this once, on a small cake, and my only suggestion would be to replace only 1/2 - 2/3 of the water, instead of all of it. The flavour was a bit smack-you-in-the-face-blueberry. I used a blueberry filling, so it was too much. If you are using just a plain vanilla filling, it would probably be fine. HTH!
Here is a link to the cake mix doctors recipe if anyone is interested.
http://www.cookingcache.com/dessert/blueberrymuffincake.shtml?rdid=rc1
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