Elvis' first hit in the South was That's All Right/Blue Moon Of Kentucky. Heartbreak Hotel was after the move to RCA, which allowed him to get national and international exposure. Sam Phillips used the money from the sale of Elvis to RCA (for at the time, the biggest price RCA had ever paid to get a star) to get Sun Records out of debt and of course, he was able to get more exposure for the likes of Johnny Cash, Jerry Lee Lewis, Carl Perkins and Roy Orbison (all Sun talent).
The fact remains that Chuck didn't record until 1955. Buddy Holly was a country artist who hadn't even formed the Crickets yet when he saw Elvis perform in Lubbock in 1955. It was Elvis' performance in Lubbock that inspired him to become a rock'n'roll performer.
The first night that That's All Right and Blue Moon Of Kentucky were played on the radio, nobody had seen Elvis. People didn't have a clue what he looked like. People actually thought he was BLACK because of his voice. They didn't find out that he was white until he started performing on stage (and they were actually surprised about it).
The point was that Elvis broke color barriers. The radio stations who played those two songs... The country audiences hearing Blue Moon Of Kentucky thought it sounded too bluesy to be bluegrass and the blues audience hearing That's All Right thought it sounded too country to be the blues. And yet, the reactions from both audiences was so overwhelming that Elvis had to be pulled out of hiding from the dark movie theater by his parents for a radio interview. He thought he was going to be run out of town for what he did to both songs.
That's the difference and why Elvis integrated music. White people could sing with blues influences and black people could sing with country influences--both of which were integral to rock'n'roll along with gospel. After this initial fusion made possible by Elvis and his racially ambiguous voice that effortlessly combined genres naturally, artists added every musical genre they could into the mix.