2006 Volkswagen Passat problem on cold startup

When I start my car in the morning I can see black smoke from the tailpipe, it. Will last about a minute or so then clear. I have replaced the air filter and any other maintenance the car needs. Its not really a problem but I know its not rite. I am no stranger to fuel injection engines and it really makes me wonder whats causing the extra fuel? I did a lot of looking for a vacuum leak but cant find a problem. Engine runs fine even when clearing itself on a cold start, but I just cant except that to be normal. What do you think could cause a rich startup.