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.