The engine may be running a bit rich, coating the inside of the exhaust pipes with black soot (carbon). Easily seen if you look at your tail pipes.
So now we have to go back to high school chemistry class: When you combine a hydrocarbon with oxygen you should get carbon dioxide and water. If you are running rich, you are going to get carbon, carbon dioxide and water.
When you first start your car, the exhaust system is cold, so the water in your exhaust condenses from steam to a liquid state and the water droplets can travel all the way out to the end of the cold tailpipes, collecting carbon soot from the interior walls along the way. The result is the spray of sooty black water droplets on the garage floor and cabinet behind the tailpipes ( and the shins of your khaki's if you happen to walk around behind your car when it is idling!).
