Blue Smoke – coming out of the exhaust pipe signifies Oil burning.
It occurs when the engine becomes old and due to the worn out piston rings, that are unable to stop the oil from reaching the combustion chamber, which burns with the fuel air mixture.
Black Smoke – coming out of the exhaust pipe signifies Rich Mixture, being used.
On ground, this can occur while starting the engine, due to over priming (priming means allowing the fuel to enter the carburetor in the liquid form, without being vaporized), this lasts for a very short while and is usually gone, as soon as the mixture in the carburetor is burnt.
In flight this can occur while flying at very high altitudes, where the air density is low and the amount of fuel entering the carburetor is high as compared to air density, the fuel contains carbon which remains un burnt and comes out of the exhaust pipe in the form of black trails. Therefore it is a general practice to lean the mixture at a specified altitude by the A/c manufacturer, which is generally 5,000 feet above mean sea level. Some manufacturer recommends the mixture to be leaned even for Take off on a high altitude Runway with high density altitude on a very hot day, since it gives maximum engine power and increase A/c range