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A large amount (relatively speaking) of unburned HC passes through the exhaust during cold starts. This is because the catalytic converter hasn't started working efficiently yet. Also, direct injected gasoline engines have higher soot/particulate emissions than port injected, especially with side-mounted direct injection. The unburned HC and the soot/particulates are where the black stuff comes from.
If you have the right gas analyzer, you can see it in realtime readings of HC concentration (ppm) or mass flow or in accumulated modal/bag data.
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