Trust Samuel Johnson to be different. The great lexicographer's house-museum just north of Fleet Street is decorated with a unique plaque. It is the only official blue plaque in the City of London. And it's brown.
The City is, of course, replete with commemorative plaques to the great and good. None of them are part of the official blue plaque scheme, save for Johnson's disc in Gough Square.
It all comes down to a definition — rather appropriately given the Johnson connection. To count as an official Blue Plaque, a memorial disc must have been erected by one of the four organisations to run the scheme: The Society of Arts (1866–1901), the London County Council (1901–1965), the Greater London Council (1965–1986) and English Heritage (1986 to present).
Johnson's plaque was cemented to its wall in 1876. This was during the tenure of the Society of Arts (now the Royal Society of Arts), who favoured brown plaques.
Shortly thereafter, the society came to an agreement that any commemorations within the Square Mile would be handled by the City of London Corporation. Johnson's was the only one to have been erected in the City up to that point.
The Corporation has since installed dozens of its own plaques, which commemorate buildings and events as often as people. These handsome adornments happen to be blue, but they are not 'Blue Plaques' in the strictest sense.
The City remains off-limits to the scheme. That said, there are some close calls. The plaque to 'Tubby' Clayton in Trinity Square is in Tower Hamlets, but only by a matter of metres. Similarly, William Caslon's memorial is in Islington by just a few paces.
If the City ever extends its boundaries again, then this piece of trivia may become obsolete, just as the claim that there are no roads in the Square Mile ceased to be true in 1994.