POP MUSIC REVIEW : James Cotton Turns In a Lively First Set of Blues at Hamptons, But Second Half Is All Downhill : Performance: After intermission, the crowd-pleasing musician seemed to become a victim of his own inner blues.

Some blues are made for the stage and some blues are probably best left private.

James Cotton’s audience heard it both ways Friday at Hamptons in a night of blues that became very strange and sad but was never less than authentic.

In his opening set, a playful, assured Cotton took his first-rate band and a responsive, hooting and hollering crowd on an energetic 80-minute excursion through traditional Chicago blues.

Then, after a half-hour intermission, Cotton, 54, returned transformed from a toying, crowd-pleasing master into an impaired victim of his own inner blues. The stage blues, in which pains and pleasures are shaped by art and filtered through a screen of performing craft, were replaced by the real-life blues, which are just plain sad. With his voice cracked and slurring, Cotton, apparently drunk, moved haltingly and distractedly through a painful and shapeless 45-minute second set.

It was very sad but not without value. As Cotton’s performance deteriorated and the veneer of stage convention came off, it left only the pure, unrehearsed expression of feeling. Cotton lurched through the private kind of blues you sing for yourself when you’re alone and hurting. The kind that an audience shouldn’t see.

A painful but arresting immediacy came through as Cotton sloppily acted out “Stormy Monday.” The audience stayed behind him, calling out in response to the lyrics as he fell theatrically to one knee. Cotton’s band stayed with him, too. It kept a rhythmic pulse going when the bandleader would halt for no good reason, and members would add vocal or instrumental fills to maintain the shape of a song when Cotton would trail off from singing or playing his harmonica.

“Sometimes we don’t give a (damn), but that’s the way it goes,” Cotton told the audience as he paused during one of the lugubrious slow blues that dominated the second set. When Cotton took a break shortly after that, guitarist Luther Tucker, a longtime veteran of Cotton’s bands, took over on a deep, taut, dolorous blues. It showed that he and the three other backing players did indeed give a damn--both for Cotton, and for the blues’ expressive power. What they were feeling at the moment--which could not have been very good--they channeled into a somber number about loneliness and pain.

Cotton’s first set was the proper gauge of his talent. He moved zestfully through standards such as “Got My Mojo Working,” “Eyesight to the Blind” and “Honest, I Do,” and an array of other basic blues numbers that were mostly up-tempo and humorous. Not the most forceful or supple singer, Cotton had trouble competing with his band at times. But most of his vocals were spirited and effective--perhaps not a technical marvel but nevertheless full of husky authenticity and pleasure.

Cotton, in any case, does his most memorable singing with the harmonica. For much of the set he adopted a coy, thin, darting piccolo tone, as if he were blowing on a blade of grass. Cotton made a great show of dynamics, backing away from the microphone to play his harmonica without amplification, then moving forward to inject a brief, throaty blast for the sake of contrast. Doing shuffling dance steps, waggling a leg, or slapping at his harp with a theatrical flourish, Cotton was a hammy but engaging showman. When he hit on a number that really ignited the crowd, he stretched it out with vocal and harp improvisations, prolonging that moment of interactive enjoyment as long as he could.

Guitarists Tucker and Bob Margolin were solid players with contrasting tones (Tucker’s was pointed and nimble like B.B. King’s, while Margolin’s solos were thick and meaty). It would have helped, though, if the stripped-down band had an extra dimension--maybe the horn section or piano player that Cotton has included in past lineups.

Drummer Ray Allison lent a dimension all his own with a loose-jointed percussion attack that was consistently exciting and varied, with Ike Anderson’s solid bass work serving as the rhythm’s reliable anchor. Allison was a smiling, shouting performer who brimmed over with obvious pleasure in playing.