We could argue endlessly about what is and isn't blues, couldn't we? 
I know for a fact that a lot of the music I post here would certainly not be considered strictly blues by most blues fans. However, if you are a regular visitor you know I play fast and loose with the blues rules, whatever the blues rules might be and that I'm not particularly interested in someone else's rules. To my ears, it all ties together. It all connects at some point. 

I'm interested in emotional content, which is certainly a basis for blues, and i'm interested in possibility and "what if."  I'm interested in the inner (and outer) connectivity of music. For example, the theory proposed by Yale professor Willie Ruff that African-American gospel music, or spirituals has Scotland as it's source, rather than Africa or slavery. That singing heard coming from the churches or the Christian Brush Arbor or revival camp meetings of the plantation owners would have influenced their slaves, who then took what they heard and made it their own.  
My point being, is Sacred Harp or Shape Note singing (i'm going to use the term Sacred Harp) any less powerful, any less moving than say, Son House singing "Grinning In Your Face"?  Certainly not to my ears. Is it blues?  Certainly not in the traditional sense. But I believe if you simply listen to the music, you'll have an idea of what I'm pointing at. 

I'm not sure where I first came across a capella Sacred Harp singing but I do know that once I researched it I was moved by the uniqueness and the power of the sound.  Sacred Harp, as Wikipedia tells us is, "a tradition of sacred choral music that took root in the Southern region of the United States. It is part of the larger tradition of shape note music." And Shape Note singing is, "a music notation designed to facilitate congregational and community singing. The notation, introduced in 1801, became a popular teaching device in American singing schools. Shapes were added to the note heads in written music to help singers find pitches within major and minor scales without the use of more complex information found in key signatures on the staff. Shape notes of various kinds have been used for over two centuries in a variety of music traditions, mostly sacred but also secular, originating in New England, practiced primarily in the Southern region of the United States for many years, and now experiencing a renaissance in other locations as well."  
That's your jumping-off point. Here's a fine explanation of the history of Sacred Harp singing and and how it works:

If you saw the film Cold Mountain then you have heard Sacred Harp singing.  The next bump in popularity this music received came via the excellent Sacred Harp documentary (and ensuing soundtracks)  "Awake, My Soul."

There are a number of Scared Harp singing groups and associations through out the country, most of them located on the east coast and south east.  There are also Shape Note and Sacred Harp groups on Facebook. If you are interested in finding a Sacred Harp group near you check