Adamb has it right, this is normal as it takes a while for the engine to warm up and get complete (mostly) combustion. Also exhaust gases entering a cold exhaust system will tend to allow the water vapor to condense on the inside of the system and allow some of the soot to stick to the inside. As it warms up and dries out just a little bit of revving the engine will cause some chunks of soot to get blown out. There usually is a far amount of soot that can accumulate in the exhaust. If you have your car out on a dark hilly road at night sometimes you can see pieces of burning soot behind you when you floor it going up steep hills.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold