At the time, before the discovery of black swans in Australia in 1695, it was thought that no such animal exists. Hence the name is ironic, as if to say "there's no place like it!" 

Samuel Pepys visited the premises on Sunday 9 August 1668, as recorded by his diary: "Up, and walked to Holborne, where got John Powell’s coach at the Black Swan, and he attended me at St. James’s..."

Fine, minor surface roughness. Very rare. 

BW 1395; Norweb -