James Blunt song "So Long Jimmy" based upon Jim Morrison?

Answer 1

Opinions being what they are, he could be referring to Morrison...or Hendrix! The lyrics very much hint at both, so it's up to interpretation.

"Digging how the guitar goes"..all Hendrix there
"Or did the voodoo magic help"..Hendrix' "Voodoo Child", but everyone knows Morrison dabbled in it.
"Does everyone get hypnotized by your fire"...Morrison's "Light My Fire", and Hendrix used to set his guitar on fire.
"Sure we're glad for the experience"...Hendrix, "Are You Experienced?"

At the end of the song, you can unmistakingly hear "Riders on the Storm"
The curious thing, is the name. Hendrix was Jimi, Morrison was simply Jim. Splitting hairs here??
So, take your pick...I think he's referring to both!!

EDIT: Just found this on Blunts website..
"His current favourite listening is Cat Power and Lou Reed's "Transformer" album, and "Back to Bedlam" has a similarly enigmatic quality. He won't explain what most of the songs are about, though he does admit that the deceptively bubbly "So Long, Jimmy" was inspired by Messrs Hendrix and Morrison. As for the rest, he says only, "You can get away with murder in a song"."

So there!!

Answer 2

That song is dedicated to Jim Morrison, that's what James' say