From a little internet search, it looks like blueberries contain between 4% and 15% sugar, 6lbs of fresh would add between .25 and 1 lb of extra fermentable sugars. So this could have a large or negligible effect on the gravity of the beer depending on what your base beer is.
It shouldn't make the beer "sweet" in that the the remaining yeast in secondary should eat right through the the fermentables in the blueberries and dry the beer out a bit. That said maybe there are some sugars in blueberries that yeast can't break down and you may get some residual sweetness. You should at least get some of the aroma, tartness, and certainly the color of the blueberries. I've heard that freezing and then thawing makes them easier for yeast to break down.
Hopefully someone else will have more experience with blueberries. Good luck!