Apparently Dickens wasn't by any means the only well known name involved in this advertiseing campaign, either writing the parodies, or being parodied, - here is a page all about it: 'The Praise of Blacking': William Frederick Deacon's Warreniana and Early Nineteenth-century Advertising-related Parody. And here is an extract from a parody of Wordsworth taken from that page:

It chanced one summer morn I passed the clefts
Of Silver-How, and turning to the left,
Fast by the blacksmith's shop, two doors beyond
Old Stubb's, the tart-woman's, approached a glen
Secluded as a coy nun from the world.
Beauteous it was but lonesome, and while I
Leaped up for joy to think that earth was good
And lusty in her boyhood, I beheld
Graven on the tawny rock these magic words,
"BUY WARREN'S BLACKING;"

I think that is pretty good!