Ceased trading around 1973. I would guess that it was named after blue man Robert Johnson.
" The singles stall was run by two separate charachters John Thorpe a long time blues fan who once ran a shop called The Sky Is Crying. His partner was Graham Stapleton, now a headmaster and brother-in- law to Cat Stevens. He comes down the 100 Club for any live acts or the Xmas party and is involved in Roger Stewart's shop Off Broadway."