The main reason is because the audience for what we generally consider to be the blues is different from what it was in the early 50s. By the late 50s the blues, as then played by Muddy Waters and others have become a traditional form transformed and revived by electrical instruments and was mainly played and clubs.
But the fact that Muddy and others had started playing on electric instruments was to have a revolutionary effect, because it eventually became early rock 'n roll, which back then had primarily black audience.
By the early 60s, white artists had started performing it, and singers like a then young Pat Boone (believe it or not) began to put it into the mainstream, albeit with different lyrics.
The 60s folk revival also played a role in that they brought many of the old 20s and 30s folk blues artist out of retirement, and again mainly to white audiences.
What really changed blues was it's rediscovery in England, which became the seed from which the "English invasion" occurred. That's another story and a very involved one.
If you take a look at the old 20s Delta blues, it's not that much different from today's rap music in approach and attitude, or at least the rap music that's not being used in commercials and versions that haven't been softened up for mass consumption.
Both early blues and rap songs were constructed the same way. A rhythmic music was produced, very much like what we would call a "vamp" today, and the singer would simply tell a story or improvise lyrics over it.
If you listen the 20s music in any detail, you find that the subject matter was actually pretty much the same as rap, just less vulgar, but the subject matter still have lots of references to party substances, sex, and had pretty much the same attitude towards women.
That's probably why so many female blue singers sang a lot about lowdown men, and a large percentage were lesbian or bisexual.
Let's take the narrative back to the 20s, and keep in mind that for space limitations, I do sometimes have to oversimplify the narrative.
The 20s is considered the beginning of the blues, but it's important to realize that black music had existed easily for over a century prior.
The primary forms of black music in the very early days was banjo music, string band music with the fiddle as the lead instrument, spirituals. ragtime, and black performers often sang the popular tunes the day also.
The reason the 20s is important is because that's when it began to be recorded. What we called early blues now was known by a variety of names including juke music, stink, and other colorful terms.
You could say it was an early form of rock 'n roll. The mainstream black church back then didn't like it, and often referred to it as the devil's music.
The "blues" back then was most commonly used as a jazz term. Basically it was a song, any song, that used the blues scale, in which two of the eight notes of the scale were flatted. That's why a lot of "blues" by jazz bands were often up-tempo dance numbers.
The 20s blues was part of a much larger music scene that the mainstream black audiences listened to. Which is the important point.
We can pretty much just fast forward to the 50s when the blues started to become a little more passé because of the rising popularity of Doo-Wop and R&B. There were a lot of gray areas of course, but I'm talking about general trends.
By the 60s the black audience pretty much moved on, except in rural areas and in the cities where a small percentage of the black audience still preferred the rawer sounds of Muddy Waters and others. In that respect it was similar to other scenes like punk, where the size of the audience wasn't very big, but the impact of the artists was huge.
Electric blues moved from a club form to a major form in England and Europe, it's audience had become predominantly white, and the music rock influenced.
There were some exceptions. For example New Orleans blues has never really changed much, and is a classic example of a regional genre that's become ingrained in the culture there.
But in the 60s, the blues transformed into blues rock with groups like Cream interpreting the blues through more modern guitars and larger amps.
At this point the black audience had moved on to soul music, which in many ways was essentially updated gospel with modern lyrics. Labels like STAX tended to stay with the blues feel, while others like Motown strived to create a more sophisticated sound.
At this stage you could say the blues had become two different animals. It had become rock and rock 'n roll on one hand, and had evolved to a different, more sophisticated form on the other.
One of the gray areas of soul music that came out of Memphis. STAX recorded a series of artists, including blues artists, and had a very definite blues feel in much of their catalog. Others like Hi-Tone which had Al Green as it's biggest star, had an early sound that was definitely blues influenced. In fact Al Green's version of "Driving Wheel,"is still one of the most popular arrangements of that standard performed by blues bands even today.
By the 70s the music industry was very big, and it was hard to keep track of all the genres.
One genre that did emerge was rap. When it first started out it was actually quite musically oriented, from the streets but like Doo-Wop, a definite new genre of music, and had hits with groups like the Sugar Hill Gang that had typical but improvised lyrics.
Rap was a street music, and its predecessor was probably groups like the Last Poets who combined jazz and poetry.
The main thing was you set up a beat, and improvised your lyrics over it, and most of it to the same melody or chant.
Now the old Delta bluesmen didn't have beat machines, so they used guitars and sometimes rhythm instruments like washboards and small drumsets if available. They would just start playing a guitar line, often on slide, and in most cases just simply begin singing.
Sometimes they would start off with one of the more popular blues numbers that were around, but frequently changed the words and as the song developed even the subject matter.
The technology was different, but the attitude, and basic theory music was pretty much the same as rap. It was a story over rhythmic music.
Even when rap moved into the gangsta style, it stayed fairly similar to the same sensibilities of Delta blues. Gangsta rappers may have sung about killing people, but plenty of Delta blues did so also, not to mention killing women who were cheated on them, and so on.
Like the early blues, it thoroughly succeeded in pissing off and offending a large segment of the mainstream population.
There were also plenty of drug and reefer songs back then in the 20s, even songs about homosexuality, songs about bad women, being a soul going to hell, and of course, all the lively and cheerful subjects that make up most of the modern blues reissues.
The raw early blues did experience a revival among the younger crowd when people like Jack White's White Stripes and the Black Keys played old blues songs on cheap guitars played through big amps at extremely loud volume. It didn't cause a revival of that style of blues, but certainly sounded good and was a lot of fun.
So is rap music a case of coming full circle from the early blues?
From a pure musicologist standpoint, and I should add I'm not a trained musicologist, the answer would have to be yes.
But the other question is, is it blues?
The answer to that is probably no, what we called the blues is now a number of sub genres from artists who play it straight to those who mix other elements into it.
In other words the blues is a lot of things now, but if you take both the white and black audiences into account, Muddy Waters statement now should be paraphrased into the blues had two babies (not just one), and they called it rock 'n roll, and rap.