Is this a straw man incident? You set up a false premise and demolished it with vigour. Why?

If, as I have always supposed, that blues was an amalgam of remembered African cultural heritage and taught western hymnal stuff on plantations, then blues will have originated wherever there were plantations.

Before the start of the 20th century the only records of this will be in written material. Letters from people who visited plantations and heard music being played, maybe travelling ministers keeping personal records or diaries kept by literate people who lived nearby.

The first slaves were in America in the 17th Century. I would expect that the music we now call Blues has been brewing ever since then.