This passage has to be read in context, so if you read the entire passage, which says:
"The coaches are numerous, and, what is much worse, there are an infinity of one-horse cabriolets, which are driven by young men of fashion and their imitators, alike fools, with such rapidity as to be real nuisances, and render the streets exceedingly dangerous, without an incessant caution. I saw a poor child run over and probably killed, and have been myself many times blackened with the mud of the kennels."
Thus it's clear that the author is explaining about he has fallen down in the streets due to the speeding coaches and thus ending up blackening himself because of the dirt in the streets.
