I did a raspberry wheat a while back (4 lbs of frozen raspberry dumped in primary). Everything was still after about a week in primary (which included an explosive fermentation of wheat+fruit), so I racked to secondary. After about a week in secondary, I started seeing a fairly constant stream of those tiny bubbles make their way up the carboy. It did this off and on for the next week even until I bottled. I didn't have overcarbonated beer or anything, so I wouldn't worry about it.
I always attributed the little bubbles to the addition of the fruit. Even though the fruit creates a VERY active fermentation at first, I figure that after that initial explosion, there are residual fermentables in the fruit that will take a long time to ferment out. I could be way off though....
EDIT: Nevermind- I read your post again and saw that you're using a blueberry extract- so there goes my idea. Maybe this is something that happens with wheats...
First one I've had do this. The recipe Blueberry Wheat; 7lbs Wheat LME, .25lbs Carafoam, 1oz Hallertau, Saftbrew T-58 dry yeast, OG 1.049. 8 day primary 66-68F. Racked onto 2oz blueberry extract SG 1.016. Noticed scummy brown islands on top of beer in primary at transfer to 2ndary. I'd never seen that before(1st clue). It started bubbling the next day and hasn't stopped. I moved it into my pantry and it got a little cool 62-64F, so I moved it to a little bit warmer temps 68F. Now it's on my 2nd floor at about 70F. I'm hoping the heat will finish it up. I brewed a witbier last summer that didn't turn out so great and this is only my 2nd wheat brew. I tasted it and checked the gravity about 10 & 17 days ago and it tasted good(although a bit sweet). The SG was constant at both checks at 1.012. The only thing flippin me out is these little tiny bubbles running up the sides of the BB. I've got time to wait this batch out. I think I transfered too early and that was my yeast sittin on top at transfer. What yall think? 
