Joan Armatrading: ''Joan Armatrading.'' A & M SP-4588--Joan Armatrading, her voice sliding from thick alto to delicate soprato, weaves a haunting tapestry of love songs in this, her first album. It is a sultry concoction of folk, rock, reggae, and blues, with obvious roots in her Jamaican heritage. What seems key to Armatrading's small but loyal following is her strength - in vocals that are as gripping live as they are on this recording, in her solid lyrics, and through her acoustic guitar riffs. Two cuts, ''Help Yourself'' and ''Love and Affection, '' capture her full range, while ''Tall in the Saddle'' leans toward blues and ''Save Me'' offers unique, almost keening vocals. Don't expect to hear Armatrading on the commercial airwaves. She's an obscure gem from the recording backwaters. This album is the find you'll enjoy stashing away - its value is in its rarity.