blues was with rock from the start. rock began as a mix of blues and southern gospel.
shortly thereafter...
when the white guys got into the act (elvis, buddy, jerry lee, etcetera) they added country. the white guys didn't know blues very well, but they knew country, so they basically switched the two, thus we have rockabilly. therefore rockabilly is a mix of southern gospel and country, with very little (although some) blues influence
some time later...
somebody decided that they were sick of the way older people looked down on rock, so they took an "if you can't beat them, join them" approach, and started to add classical influences. thus we are forever cursed with pretentious "progressive" rock, and obligatory orchestral performance cd's when bands get too big. this began some time in the mid sixties, and may even be the fault of the otherwise faultless Beatles.
a hop skip and a jump ahead...
rap, like rock, began as a medium of poor black kids, who couldn't afford musical instruments. turntables on the other hand were cheap (and already onwed by the perspective rappers family). so they developed scratching and sampling.