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: The smoke problem began right after the car was driven in reverse, while turning (steering clockwise), down the lip of the driveway, with the muffler striking the pavement hard and scraping for a couple of seconds. The unwanted impact tried to push it (and, presumably, the connected exhaust pipes), toward the front of the car. Along with the smoke problem, noticeable loss of horsepower occurred.
Currently the muffler has an old, corroded exterior, with a good-size laceration hole in the outer metal shell (I can put a couple of fingers through the hole). 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