Jonathon Blott, oil and colourman, was recorded in the 1881 census as living at 484 Bethnal Green Road with his 34 year old wife Eliza, who was born in Elmham, Norfolk.
The identification of Jonathon Blott as the 'Mr Blott' concerned is not absolutely certain since the 1881 census also records a Samuel Blott, a 55 year old auctioneer's assistant born in Leighton, Huntingdonshire, and living at 124 Cleveland Street with his wife Sarah Ann (then aged 34 and born at Watford).
However, since Jonathon Blott was a colourman it seems more likely that he was the one known to JW.
Blott appears to have been in contact with both JW and C. A. Howell in the 1870s, and to have lent money on the security of JW's pictures. He may also have been an art dealer.
UK census 1881, from http://www.familysearch.org (accessed 2004).