I've seen / read many things about black music, especially the Negro spiritual, blues, rhythm'n'blues, rock'n'roll, etc.
And 99% of what I've heard or read says that the rap music descends from blues, for example.
I love hip hop culture, but I can not understand the musical relationship with blues.
Does anyone know how the two are related?
There's a hip hop relationship with the blues, but perhaps a distant one and not too evident.
There aren't many traces, if any, of the more traditional blues in hip hop. No 12 bar form and no blues harmonic structure (hip hop harmonic structure is usually fairly flat and static).
But the more primitive forms of blues where also rather static harmonically, for example when performed with the single string instrument the diddley bow (example). Also, one of the more direct musical influences of hip hop is the "turntablism" of Jamaican origin, that in turn arised from reggae (or at least from the reggae culture), that also had some blues influences.
Regarding lyrics, usually you don't find in hip hop the AAB standard blues form. But there's a close relationship with the oral southern black tradition that is found in the toast oral poems and primitive blues with story telling traditional lyrics. Some form of call and response can often be found in hip hop, and that's another common point with the blues, but could come from many diverse influences. Many other direct and indirect influences, ancient and modern also exist (see the wikipedia rap article).
Thematically and sociologically, I guess we could say that hip hop has today for a certain community a similar role to what the blues had at the end of 19th and most of 20th century, despite the obvious differences between the two forms. It's a cultural manifestation by which a commmunity expresses their problems under a story telling guise.
This is mainly intended as a supplement to @JoséDavid's excellent answer.
Nearly every indigenous American musical form (other than Native American music) descends or was in some way influenced by the blues --it's the root of the family tree of American music. However, hip-hop is pretty far distanced from that ancestor --it's the child of rock / R&B, which are themselves the children of the blues, and much of what hip-hop retains from rock is not the material that rock got from the blues.
On the other hand, many of the edgier old blues songs have a certain similarity in attitude to hip-hop --there's a similar braggadocio, casual attitude towards violence, and celebration of raw sexuality (compare, for instance, Li'l Kim's "Magic Stick" with Clara Smith's "Tight Like That"). It's also the case that both rely more on clever lyrics than melodic development --many blues songs have essentially identical melodies.
I see a direct lineage from Blues and Gospel to Rock'n'Roll, Soul, Funk and Rap.
This first is an example of the time old tradition of 'playin' the dozens'
Bo Diddley "Say Man" Hookers spoken interjections was a part of his repertoire as well as many other blues artists throughout history. John Lee Hooker "Boogie Chillen'" Those are two examples of rap in blues but if you really want to hear how rap evolved from blues roots watch this 4 minute video to the end. Reverend Robert Jones Roots To Rap.I mention Gospel because the preaching aspect is so important to the genesis of blues. (pardon the pun).
This is my perspective on it. There are other examples but I don't think it's fair to discount the deep history and impact of Blues and Gospel in the emergence of rap. Hip Hop borrows heavily from Funk and Rock (both of which have their roots in Blues and Gospel) in it's rhythmic aspect as well.
It could be said that hip-hop is not as much musically influenced by blues as it is emotionally. Blues comes from the songs sung by African-American slaves, and because of that it is painful, soulful and very human. It was a slaves way of expressing his human feeling in the midst of being treated in a sub human way. Hip-hop similarly came out of poor, generally black communities, whose inhabitants were probably literally the descendants of slaves. In these neighborhoods, violence and drug use, etc. were very common. A person living in these conditions no doubt felt oppressed, or scared and hopeless. So the music that came out of it was fueled by the very same emotions that drove the slaves to sing of their pain. Its the purpose of hiphop lyrics to express this pain and suffering (every aspect of the "hip-hop" lifestyle) and that is what makes it a descendant, if not a form of the blues. The blues is not just a style of music, it is an attitude or a situation, and it is something everyone can relate to sometimes. everyone has good days and bad days. This is probably why hip-hop has become so popular; it can resonate with almost everybody in some way or another.
Sorry for my potentially less educated guess than those who have posted before me but I believe it's very evident that rap was influenced by hip hop, and slightly less evident I believe that hip hop was influenced by jazz, via the structure of it's poetry and symbolism, from there we can trace jazz back to its roots in blues directly
