It's an excellent idea. I myself will be finishing up a sour (Lacto) blueberry brown ale in the next couple months. The sour brown is already done -- the blueberry, because I just so happen to have about 5 lbs blueberries in the freezer right now.
Don't use any extracts or liquours in your beer, they're nasty. Use real fruit, frozen ahead of time to have the ice crystals bust up the cell walls on the microscopic level. Canned puree should also work real well. Just make sure it doesn't have preservatives added. We can add our own means of sanitization of the fruit...
For a subtle flavor, you'll need about 1 lb fruit per gallon. For a bolder flavor, try 2 lbs. With raspberries, I would use no more than 2 lb, which is kind of a lot honestly. With blueberries, I wish I had more because I'd like to use 3 lb per gallon because they are not as strongly flavored as raspberries. Maybe I'll split the batch and do 2 or 3 different fruits, I dunno yet.
Ferment out the beer as normal till done or almost done. Then pull the fruit from the freezer to thaw, add 1 crushed Campden tablet per pound, mix well, and set back into the refrigerator for a couple of days. This will kill any nasties. Then set the treated fruit into a fresh sanitized vessel, and rack the beer on top. Then let this ferment for a good couple of weeks or until done.
That's how I'll do it. I'll even rack the beer to tertiary at the end and let settle for several more days to keep most of the chunks out of the bottles.
Keep in mind, any fruited beer will have a tendency to be VERY tart indeed, and a bit thin too since the sugars are like 95% fermentable. Hope you don't mind those things. If you find the finished beer too tart, add 1/2 to 1 lb lactose per 5 gallon batch size to help balance the tartness with a little sweetness and body.