Believe it or not: Weight saving! F1 teams use dark grey/black carbon fibre to build their cars and so by having a black paint scheme they use less paint to cover the base carbon fibre for the livery. If you look at Ferrari's recent liveries you'll notice that the cars are no longer all red - A large part of the car's livery these days is black (in fact it's left in bare carbon fibre with no paint at all!). By using black paint/no paint at all, teams can shave 0.5~1.5Kg of weight from the cars. They can then use that saved weight as moveable ballast to get a better balance on the cars chassis. (When McLaren finally decided to ditch their Chrome paint livery for a plainer black livery, they instantly freed up @ 3Kg of weight!)
+1 My 'era'. The damn tires were heavy all by themselves.... JPJ was quite the jockey too. Never in the right place at the right time. Helluva driver. Cheers, Ian