If you've never heard the legendary Blind Boys of Alabama, by all means take the opportunity when they come to the Ark on Monday, November 17. Formed as the Five Blind Boys of Alabama in the 1940s, they cultivated a sound unlike any of the other African American gospel harmony groups of that era — unlike anything you've ever heard, really.

They had all the agility displayed in the smooth harmonies of groups like the Swan Silvertones and Sam Cooke's Soul Stirrers. But their lead vocalist, Clarence Fountain, was a rough-edged shouter, and the others in the group could match him and cut loose with the unbridled sounds of the preacher at full throttle. The group as a whole, then, slid not only from pitch to pitch and harmony to harmony but also from timbre to timbre, creating gospel harmony with a wallop.

These days, Fountain sings lead on only a few numbers, offering quiet statements that have the quality of wise asides. Younger singers who can evoke his power have been brought on board, and even as the number of Blind Boys fluctuates, the group's harmonies sound as strong as ever. Another compelling reason to hear today's Blind Boys of Alabama, though, lies in what's changed rather than in what's remained the same. Whereas the other classic gospel harmonists are long gone, the Blind Boys of Alabama have reinvented themselves while keeping their core sound undiluted.

They've kept their music vital by stretching themselves through collaboration. The process began with a stage musical called The Gospel at Colonus, a black gospel adaptation of Sophocles whose 1999 revival at the Power Center remains fresh in many Ann Arborites' memories. The Blind Boys have also toured with bluegrass musicians and recorded with gospel steel guitarists.

On Go Tell It on the Mountain, their first Christmas album, the Blind Boys share lead vocals with a variety of guest stars, from Solomon Burke to Chrissie Hynde. The group's amazing vocal harmonies basically absorb whatever ingredients are added to them; it's just about impossible to make these singers into background vocalists. Several classic African American singers — Aaron Neville, Mavis Staples — add counterpoints that build on the original textures of the Blind Boys' music.

These various guest stars won't be present at the Ark. What will be retained are the arrangements the Blind Boys made of classic Christmas carols, and of contemporary Christmas pieces by the likes of Harry Connick Jr., to accommodate the modern vocalists. Those arrangements draw on much of the music — soul, rock, jazz, and more — that has come along since the Five Blind Boys' 1950s heyday. Chrissie Hynde and the others added little, really, except a dash of familiar flavor. The miracle of the present-day Blind Boys of Alabama is that by hearing them, you can get in touch with something that comes from a very old place, a tradition that has grown to encompass the new.