Lighter. What makes you say that? As I mentioned, the blues first emerged around the turn of the 20th century (see for example Paul Oliver , Songsters and Saints). If we're looking for social causes, such a late date would rule out the direct effects of slavery, and BTW it would also rule out the notion of the Blues as an African survival.
Therefore, the social conditions which produced the Blues must have arisen sometime during the last two decades or so of the nineteenth century. IE., precisely when the earliest Blues singers were growing up and suffering the effects, not of slavery, but of the White backlash.
If there was something else going on then, besides the Ku Klux Klan, lynchings etc., then I appear to have missed it. Perhaps you could enlighten me.
Just on the score of the Blues scale, I am not a musicologist and I don't know my way around the various mucial scales to be heard in Africa. However, it's worth pointing out that the Blues scale is in fact the Dorian mode with a extra flattened note. Please don't quote me, but wouldn't it be surprising if the roots of the Blues lay not in Africa, but in the sounds which Black singers heard their White neigbours making? The high lonesome sound and all that?