Most Spirituals have as their antecedents the "old time" music that was widespread in this country before the Civil War, a common well-spring that gave rise to both black and white gospel music. And both of these gospel forms seem to have more in common with each other than with the Blues forms that arose around the beginning of the 20th Century, both from a structural and thematic standpoint. The twelve bar Blues formula is not in evidence in black spiritual music that pre-dates it, and the cultural good-evil, gospel-blues antithesis kept most early blues from cross-breeding with spiritual music. It's certainly more likely today that there are blues-gospel crossovers, but I believe this to be a recent development.