It starts, actually, with the pentatonic melodies of traditional Native American musics. You'd think Africa would be the starting place, but, after a while, the Nth-generation slaves had more immediate contact with Indians than they did with the Motherland (e.g. there were also Indian slaves in some locales, and there were periods in which runaway slaves found safehouses amongst the Native communities). Africa was just ancestral-memory stuff. The one innovation that probably came from there was rhythmic - syncopation and a generally freer pulse.

But while the indigenous US population suffered extermination and/or assimilation over the years, black culture was allowed to develop on its own - thus begins the forking of the blues. The field holler may be the closest thing to the blues' Indian roots (an aside: the watermelon salesman).

By the 19th Century, some black slaves had found special "house nigger" positions, via sports (e.g. horse racing - this is the origin of the odious "lawn jockey" decoration), and via music. There would soon be white composers and musicians imitating this new music, creating all sorts of forks in mainstream popular culture over the decades.

Another change from Native roots was the Western song form, probably brought about via another fork - the spirituals, the origins of gospel music. This presumably also introduced Western instrumentation into the mix.

The 20th Century begins the forking that leads to the multifaceted connotations that we now have when one says "blues". The jazz fork starts, arguably, in New Orleans, but a blues in jazz can now mean any number of things, since jazz (the fork most willing and able to incorporate innovation from European music) has been a venue of increasing complexity ever since the days of Buddy Bolden.

The guitar became the signature instrument of non-jazz blues, and, like the saxophone in jazz, where it eventually took over from the trumpet as King of the Instruments, it lent itself more readily to non-Western applications - neither Adolphe Sax nor the luthiers of olde had any idea of what would become of their creations.

To paraphrase blues master Sean "Puffy" Combs, it's all about the blue notes - the third and seventh degrees of the major scale, altered. The textbook definition might say "lowered", lowered to resemble the third and seventh degrees of the minor scale, but that either/or, major/minor dichotomy is a creation of the European tempered scale; in the rest of the world, musics had the freedom to hit those points in between. Fuzzy temperament, if you will. Indian (as in India) music, Chinese music, North African music, etc, etc, each have their own indigenous ways of being fuzzy - the blue notes (and you can include the flatted fifth degree as well) are the North American form. In blues, the sliding of a bottleneck along the guitar strings or the bending of those strings became a way to approximate the fuzzy movements that a good singer was capable of doing.

More forks: country music (which was always multicultural); Chicago blues, where rural Southern acoustic bluesicians moved North (and West, to places like California) - especially after World War II - and added heavy amplification, even to the harmonica. And rhythm and blues, which may have been, in part, a fork in jazz - the art-music people gravitated to bebop, while R&B kept closer to the dance-music simplicity of earlier eras. The jazz-based "jump blues" of the 30s/40s segued nicely into the R&B of the 50s.

By this point forks were already merging with other forks (and creating new ones), so it becomes silly to try to keep up with all this. Plus I've only covered one thread of what "the blues" is. I leave it to others to flesh this thing out.

Of course, I may have made this all up. All music is rock and roll anyway.

A shout-out to Professor Heretic, who made possible this writeup.

Next: the Puerto Rican and Jamaican origins of hip-hop.