Throughout his career, Van Morrison has been known as an extraordinarily honest and direct singer, as a performer who holds nothing back. So when he emerges from nearly three years of self-imposed silence with a new album and decides to call it "A Period of Transition," you can be sure that he's embarking on a dramatic shift away from the eclectic style that made him one of pop music's most acclaimed vocalists.

What the album title doesn't explain is: a transition to what? It's only once the record is on the turntable that the answer emerges, that it becomes obvious Morrison has gone back to his roots. Produced by Mac "Dr. John" Rebennack, the New Orleans pianist and funkmeister whom rock luminaries regularly turn to when they'r seeking the extra touch of soul, "A Period of Transition" Warner Bros. BS 2987) is an exercise in basic, down-to-earth rhythm 'n' blues.

It may seem odd to be talking about the rhythm 'n' blues "roots" of a stocky, little red-haired fellow from Belfast, Northern Ireland, but Morrison long ago established his r 'n' b credentials. After all, it's been a good dozen years since he wrote and, as the Ray Charles-influenced lead singer of a group called Them, recorded "Gloria" and "Mystic Eyes," two classics of the genre.

And although no single tune on "A Period of Transition" is quite as raw or urgent as these two songs or "Here Comes the Night," the album as a whole is unmistakably rougher and more intense than Morrison's recent solo efforts.

Particularly impressive is the way Morrison responds to his three-man horn section. He's always been a horn-oriented vocalist, feeding off and extending a saxophone or trumpet phrase, but the give-and-take between singer Morrison and horn player Jerry Jumonville on "You Gotta Make It Through the World" and "It Fills You Up" seems especially notable.

On the latter number, which features Morrison himself on the "Mississippi saxophone," as bluesmen used to call the harmonica, the opening inspiration is a bluesy saxophone phrase. On "You Gotta Make It Through the World," the first of the album's seven songs, Morrison loosens up only after Jumonville quotes from "Night On Bald Mountain." And on "Joyous Sound," another standout tune, the exuberant Memphis-style horn lines spark Morrison's jauntiest vocal excursion.

Admittedly, Morrison's policy of simplicity and retrenchment doesn't always work. "The Eternal Kansas City," intended as a tribute to that city's crucial role in the development of jazz and blues, takes this impulse too far: by the time the choir which opens the tune by inquiring "Excuse me, do you know the way to Kansas City?" has yielded to Morrison, most listeners will have lost interest, if not patience, with the song.

But "Heavy Connection" suggests that Morrison can do as much with stock r 'n' b lyrics and with rifts. The opening line of the song - "In the land of a thousand dances I would dance with you" - is a wonderful evocation of the old Chris Kenner tune, and a little later on, he works in the phrase "from a whisper to a scream," showing that a mileage out of the most traditional themes.

Sometimes similar occurs on Delbert McClinton's "Love Rustler" (ABC AB-991), the new album by a singer whose r 'n' b background is almost as unusual as Morrison's. A white Texan who has spent time on the road with Jimmy Reed and Howlin' Wolf McClinton first was marketed as a country singer, and even now records with some of Nashville's top session men.

But with his reworking of "Ain't No More Cane," and old Texas prison song, and Bobby "Blue," Bland's "Turn On Your Lovelight," McClinton has emerged as an important new blues and rock voice. He manages to inject funk into even a country classic like "In the Jailhouse Now" and the gospelish "As Long As I Got You," and if that's not a welcome developmen, then, as Little Milton once said, "grits ain't groceries."