There's a great piece in Slate "It's All in My Head" by Jessica Winter, that attempts to make a distinction between procrastination and writer's block focusing on the work—and lack thereof—of famous writers of the past such as Truman Capote.
Here's an excerpt:
Neurologist Alice Flaherty attempts a working distinction between procrastination and block—the fearsome Orthrus of the creative process—in her 2004 book The Midnight Disease: The Drive To Write, Writer's Block, and the Creative Brain:
"A blocked writer has the discipline to stay at the desk but cannot
write. A procrastinator, on the other hand, cannot bring himself to sit
down at the desk; yet if something forces him to sit down he may write
quite fluently." But don't these two scenarios amount to different
performances of the same role? Every seasoned procrastinator loves to
tell himself that, amid his flurry of avoidance strategies—rearranging
the furniture in his office, pitching himself into a YouTube rabbit
hole, surrendering to a fit of self-Googling—his brain is secretly
marinating ideas and hatching plans. (As the underground narrator of Invisible Man
puts it, "A hibernation is a covert preparation for a more overt
action.") Surely this percolation process is also happening inside the
"blocked" writer, even if he's motionless in his swivel chair?
My goodness, think of the trouble Capote et al. would have had if they had the Internet!