Petrol and diesel are hydrocarbons. When you burn them you produce carbon dioxide and water. When the vehicle has reached running temperature the water leaves the exhaust pipe in vapour form; on a cold engine it can condense in the exhaust system and drip out.
It appears to be a dark colour when you look at it on the road , where it's dripped unto .
I guess therefore it's because the water is mixing with the 'soot' in the exhaust pipe ?
quite likely. It will also corrode the pipe if it's lying there long enough and often enough; this happens to people who don't drive long enough to heat up the pipe and vaporise the water
Bazile, the air filter is supposed to suck air through it and at high revs it is going to be sucking more air, if it's running ok don't worry about it. Or has someone fitted a cold air induction system on it ?.