You can pretty much add anything to a fermenting beer.
*PSA: please exclude eggs & dairy products as well as gardening tools, radiator fluid, old shoes and house pets from your brews. Small children are also non-fermentable.*
[ame="http://www.google.com/search?hl=en&safe=off&client=firefox-a&rls=org.mozilla:en-USfficial&hs=c4F&q=+site:homebrewtalk.com+blueberry+beer+recipe&ei=_8cFS6KsFZSKnQegqvm_Cw&sa=X&oi=forum_cluster&resnum=3&ct=more-results&ved=0CBYQrQIwAg"]Here's what Google pulled up on HBT.[/ame]
I've never added fruit to beer, but I think the time to do it is after you've transferred it to secondary. Someone else who has experience will probably pipe up here...