Countless young songwriters claim they wouldn't be doing what they do but for Joni Mitchell's Blue. Yet many of them heard it not when it came out in 1971, but years later as children, via their parents' ­record collections. It seems to have influenced them as deeply as anyone who might have had first-hand experience of its raw adult emotions; its psychic life was ­different for a young person, perhaps - more about the album's rainbow spectrum of colour and feeling.

I recall its planet-hopping locations, its dynamic men, its sharply articulated if mysterious sense of how those men made her feel. I remember, too, its lyrical quirks ("sunset pig"?), and above all its intense sense impressions - up into happiness, back down into sadness, like riding a bike over hilly land - which had as much to do with her gently gymnastic melodies and homemade chords as it did with the human dramas she wrote about.

It was a revelation to me to discover, as an adult, that Blue was written at a time of great personal fragility, and not long before a full psychological breakdown that Joni, being Joni, described as a shamanic experience. Blue did not feel fragile: it was gripping - you held your breath, as she made simple the most complex processes that take place between two people when they attempt to share a life: "I love you when I forget about me" ("All I Want").

Fifty years on, "Carey" still feels like a puff of air between your ribs, like remembering you are in love after a moment of forgetting. But it is not a love song - it's a leaving song, entirely propelled by the sense that it is set over one last night. When I was six, I assumed it was dedicated to an ancient lover ("Carey, get out your cane"). This was the same "old man" carried off by a big yellow taxi on Ladies Of The Canyon; the same old man from "My Old Man" on Blue. Why Joni was with this cantankerous old git I didn't know, but she cheered him up - renewed him again and again, as she might say.

In fact Cary Raditz was 23 when Mitchell met him in the Cretan fishing village of Matala. He had wild red hair and was hitchhiking around Europe after quitting his job in North Carolina. He cooked in Delfini's taverna: Mitchell brought her finished plate up to him one night and he threw it on the floor. Raditz pops up in "California" - "met a redneck on a Grecian isle, who did the goat dance very well" - as a near-mythical, ­Pan-like figure. His "cane" was a knobbly shepherd's crook he used for climbing up the mountainside. Mitchell lived in a cave with him for a time - a cave! - but she doesn't say that, too much of a genuine hippy to labour the point. Raditz became an investment strategist in Maryland. Every couple of years I write to him, but he politely declines to be interviewed - always adding that he ­recently saw Joni, and is working on his memoir.

Mitchell leaves Raditz, according to the song, because she misses her clean white linen and her fancy French cologne. Her irresistible urge for going, a feature of so many songs, is satirised in all those sharp, painterly impressions - what better reason to go to Amsterdam than to hire a grand piano and put flowers around one's room?

She also left her home with Graham Nash, the real subject of "My Old Man", during the writing of Blue. There are moments of domestic happiness implicit in the intensity of missing him: "The bed's too big, the frying pan's too wide", but it feels temporary somehow. Nash asked her to marry him, of course - "Our House" was his song of settled bliss. But Mitchell had been married, young at 21, when she didn't want to be: she'd walked down the aisle telling herself "I can get out of this".

People like to say that the first half of Blue is about hope and the second is all disillusionment, culminating in "The Last Time I Saw Richard", an incredible piece of telescopic psychology in which Mitchell is told that if you remain hooked on the idea of romantic love you end up cynical, boring someone in some dark café. But to focus on disillusionment is to overlook the fact that, for certain kinds of minds, love burns most brightly in the losing of it.

"A Case Of You" vibrates with this strange truth. If "Carey" is on one side of Mitchell's rainbow spectrum, its uplifting intro a psychic echo of joy and togetherness, then "A Case Of You" is at the other - alone in the blue TV light, set to dulcimer chords so thin you can hear the missing notes. The song still feels like stepping off a cliff: there is a precious heaviness about it. "You're in my blood like holy wine, you taste so bitter and so sweet." Perhaps love is not just about connection between two people. Perhaps it is something you can do on your own.

Mitchell later said that after this wide-open record, the writing of which was at times excruciating, she came to "stabilise" psychologically, but one wonders if she simply got to know herself better, too. The five years after Blue brought about an extraordinary variation on a theme as, again and again, she set the urge to find connection against the desire to flee.

On Court and Spark (1974), the unavoidable pull of romance ("Help Me") was held up against the startling image from "Free Man In Paris", "unfettered and alive". By Hejira (1976), she was a kind of warrior, singing to Amelia Earhart, with a "diamond snake around my arm". While once the wanderlust had propelled and confused her, she comes to understand, or be at peace with, the addiction. Jet trails become "the hexagram of the heavens, the strings of my guitar" - as though she'd accepted that 30,000 feet above the Earth was, for whatever reason, the place she felt most alive.