Greetings!  Wow!  Twice in one month I actually have something to contribute
to this forum other than questions!  To wit: although black smoke is indeed
unburnt diesel, it does not necessarily follow that by throttling-back to a
smoke-free condition there will be no significant reduction in power.
Drivers of semi-trailers, especially in the mountains, will confirm that
pumps are sometimes adjusted to where considerable smoking occurs to gain
the extra power.  This is because, unlike in a gasoline engine where a
near-stoichiometric (sic) mixture of air and fuel is required, diesels need
a surplus of air.  As more fuel is added combustion is not adversely
effected although a certain proportion of the fuel is not burnt.  This
proportion does increase but not linearly.  As the volume of unburnt fuel
increases smoke is the result.  However, because a proportion of the added
fuel is burnt, there is an increase in power.