Without doing some major research for you, I'd agree that gospel in some form certainly came first since blues in a recognizable form seems to be a post-1900 development, but I would venture to guess that blues had roots more in field hollers and prison songs than in gospel. There were plenty of musicians such as Thomas Dorsey and Son House who went back and forth but many church-going people, to put it mildly, frowned on blues. Later--1930's, 1940's--when blues had become more accepted among "respectable" people, the musical conventions were applied to gospel.

There is a very interesting CD available from Folkways called Black Texicans: Balladeers and Songsters of the Texas Frontier. It was a Lomax project and is mostly later recordings, but I imagine that the music could be similar to what might have been sung by slaves and Reconstruction-era sharecroppers and laborers (many of the singers on here were prisoners). Actually, there are a lot of very familiar titles and probably borrowed at least in part from European tradition--St. James Hospital, Little Liza Jane, Buffalo Skinners, Old Joe Clark--but in versions that are utterly different from the "white" ones we're used to.

Also, there are some blues artists like Mance Lipscomb who were sort of intermediary between field and dance songs and blues. Might want to look up a few of those as well.