First, I'm not quite sure about the color of the smoke: at first glance
at the tailpipe end, it seems white, but looking at the billowing cloud
you think it could pass for black. One could even suspect blue, but
that could be an illusion due to the green foliage in the background.

As to cause: my research suggests likelihood of coolant leak into the
engine (most likely through a busted head gasket). I have read that in
such cases the smoke "smells sweet". Having no experience here, i can
not be certain what "smelling sweet" is, but i would be willing to
describe this smell as sweet.

However, the facts with this car are:  According to the driver, the
smoke problem began right after the car was driven over a sidewalk
elevation whereby the muffler was impacted.  (I am not ruling out that
the sidewalk may have bumped some other part, but the driver seems
certain it was the muffler).  Currently the muffler has an old,
corroded exterior, with a good-size hole where the outer metal shell is
lacerated a couple of inches. Coinciding with the start of the smoke
problem and the impact, the car also noticeably lost horse power. There
are soft, low rumbling noises coming from the muffler area.

I do not know enough to rule in or out the possilibity that the muffler
is the one (and only) cause of the problem. Could it be a cause at all?

In case this is relevant: with a recent replacement of ignition stuff
(sparkplugs, wires, distributor) the engine sounds great and runs
smoothly. (Could it run smoothly with a blown head gasket?)

thanks

Andrew