The Fuel to Air ratio is too rich, so as the car burns petrol, there are more carbon atoms in the combustion, and rather than bonding with oxygen, they just come out as straight carbon, which is black!
Another by product of combustion when the engine is cold is water vapour, so there is an element of Steam to the exhaust gasses, and this combines with your carbon to make black looking water.
To solve the black smoke issue, it might need a mild tune of the mixtures on the carburettor. Buy a workshop manual (Gregorys) for about 25 dollars and it will give you a run down on what to fiddle with on your carb, and the steps to take.
When you get it wrong (part of the experience) a mechanic should be able to tune it properly for ~50 bucks if you talk to him nice!
Best of luck, and remember whatever you do can usually be undone!
Cheers
Tom.