May 8, 1911 – August 16, 1938
The 27 club is a group of well known musicians that have died at the age of 27. It's pertinence to the Blues is that it's founder and, arguably, it's most famous member is none other than Robert Johnson himself, who died under mysterious circumstances at the Three Forks Store in Greenwood, Mississippi at 27 years old, after claiming to have sold his soul to the Devil. Eerily, many of the musicians who have "joined the club" at 27 have directly cited Robert Johnson as a major influence. Some of the most famous members are Kurt Kobain of Nirvana, Janis Joplin, Jimi Hendrix, Jim Morrison of The Doors, Alan Wilson of Canned Heat, and Brian Jones of The Rolling Stones, all of which had very heavy Blues influences, including Johnson, and often covered Blues songs, including Johnson's. Every one of their lives parallelled that of Johnson's in certain ways, adding to the eerie coincidence.
Eric Clapton, who also cites that Robert Johnson is probably his biggest influence, was purportedly the originator of the term "27 Club", stating that he himself was almost a member when he was fighting an addiction.
Source: Bluescentric | Blues Dictionary, Blues glossary, Blues Terms, Definitions, and Meanings