Rock changes are basically Blues changes. Rock instrumentation (Guitar, Bass, Drums[, Keyboard, Horns]) is basically the same as Blues instrumentation (Guitar, Bass, Drums[, Keyboard, Horns]). Can one make an objective, quantifiable distinction between Rock-n-roll and Rhythm-n-Blues?
Short answer: No.
Alan Freed's use of the term rock-n-roll in the 1950s is often considered definitive. He used the term to refer to R&B combos, black vocal groups, saxophonists, black blues singers, and white artists playing in the authentic R&B style.
Rhythm-n-Blues was early Rock-n-roll.
Early rock-n-roll was the popular, and commercial (and often "white") face of Rhythm-n-Blues.
Whilst Wikipedia says...
Once you get towards the end of the 1950s however both terms have started to evolve along interweaving paths into RnB and the all encompassing umbrella term of Rock music.
The term "Rhythm and Blues" or "R&B" was coined by Jerry Wexler (who went on to be a famous record company executive and producer) when he was working as a journalist at Billboard Magazine, circa 1952. (Wikipedia link). Billboard published a weekly chart displaying their estimates of the relative position of retail sales of single recordings which were also broadcast on radio stations with an African-American audience, and the chart was called "Race" (as in "racially-oriented", or perhaps "oriented to the minority race" which in the United States of America meant African-Americans). Recordings made by and marketed and sold to African-Americans had previously been called "race records". Wexler proposed the name "Rhythm and Blues" at a staff meeting of Billboard magazine when they discussed wanting to change the name of the "Race" chart to something else, and his suggestion was accepted.
As was mentioned earlier, influential radio personality Alan Freed coined the term "rock and roll" circa the mid-1950s. Freed seems to have intended the usage to refer to music made by both African-American musicians and white musicians.
Quickly, though, in popular usage in the United States of America, "rock and roll" came to refer to music of a certain style recorded by white musicians and marketed and sold to a white audience. "Rhythm and Blues" refers to music that was recorded by African-American musicians and marketed and sold to an African-American audience.
Note that the key to this is the term "marketed and sold". Both "Rock and Roll" and "Rhythm and Blues" came to be designations used by the commercial music industry to describe how the music was marketed and sold, and to whom it was marketed.
Notwithstanding, there are certainly distinct artistic characteristics to each kind of music.
The rhythms used in the two styles are similar. I would note that rock and roll has usually involved simple instrumentation of bass, guitar, and drums, small ensembles, and strictly diatonic chord progressions (mostly just I, IV, V7 or bVII if mixolydian mode is used). R&B, on the other hand, freely incorporates the expanded harmonic palette of jazz chords, uses larger ensembles, and can be found frequently to incorporate keyboards, horn sections, woodwinds, and string orchestras. R&B also tends toward more mid-tempo and slow-tempo pieces. The rock singing style is straightforward and narrative, whereas the R&B singing style involves florid melisma and other embellishments. It can also be noted that female singers are much more prevalent in R&B than they are in rock, where male singers strongly predominate. There is also a lot of influence of hip-hop and rap found in R&B in the last fifteen or twenty years, whereas hip-hop and rap have a much more limited influence on rock (and country) in the same time period.
First of all' the people that are saying that rock n roll means rhythm and blues white is wrong. 100 percent wrong. Many rock n roll singers from the 50s were blacks. There were both a lot of black and white rock n roll singers. Rock n roll came straight from rhythm and blues, even Elvis said that rock n roll was here before he came, it was called rhythm and blues. Alan freed in 1951 use rock n roll to describe black rhythm and blues music to a more wider audience other than the black dominant that the genre was sold to. He was trying to hide the rhythm and blues name with using a name called rock n roll, pop radios wouldn,t play black rhythm and blues so Alan freed decided to duiguise the name as calling it Rock n roll so it can het a wide appeal. The music was all just rhythm and blues but it was having a fake name so it can appeal to more people other than blacks. The differences between rock n roll and rhythm and blues is that rock n roll is a uptempo version of it, it's faster and heavier. It also uses more need of the electric guitar although many rock n roll artists in the 50s had a saxophone for the dominant sound, some like Elvis guitarist which is Scotty Moore, Chuck berry and Buddy holly influence largely the next movement of rock music which was in the 60s. Rock n roll is a uptempo version of it, uses more need of the amplified electric guitar and in most times it's also heavier because it's more uptempo. It makes you move more. If a rock n roll artist uses more of the saxophone instrument for it's dominant sound then it should also be noted that the music is faster than rhythm and blues. However most of them have a piano mix with the saxophone but the difference is that rock n roll is played faster with the piano, the pianists playef faster than rhythm and blues. Also the big backbeat that rock n roll has that rhythm and blues lacked. Later artists like Elvis, Chuck berry and Buddy holly would use guitars to expand the instrument which later rock music would have as the dominant sound. It should also be noted that rock music is rock n roll, rock is rock n roll. It came from rhythm and blues but it is awas a faster version when it came, not you have amps being larger and bigger and more electrified and you can easily tell the difference now but back in the 50s, rock was a uptempo version of it, using more of the electric guitar if the electric guitar wasn't dominant then it was still more uptempo because of the piano.