In my experience with promoting both poker and casinos, I have never been a loser (on the house side) overall from blackjack.
If you only have 2-5 people on your tracker for casino, then yes you could lose. But it's a numbers game with casino bets. Your losers will almost always cancel out the winners in the long run though if you have a fair amount of players.
That does suck however when someone wins at BJ and wipes out your poker income.....I try to avoid those situations whenever possible, but believe me, it happens to all of us.
I really do hate mixing my poker $ with casino $ if at all possible.