Final answer:
Ray Charles is the musician who was told he could not play at a Las Vegas casino due to racial discrimination, despite playing a significant role in the popularization of rock n' roll music.
Step-by-step explanation:
The musician who booked a gig at a Las Vegas casino but was later told that he is not allowed to play at the venue because 'blacks were not allowed in the casino' is Ray Charles.
Despite the rising popularity of African American musicians and the hybridization of musical styles, segregation and racial discrimination were still rampant during the era when these artists rose to fame. Although artists like Chuck Berry and Little Richard were instrumental in the development and popularization of rock n' roll, it was Ray Charles who experienced the specific incident at a Las Vegas casino, highlighting the racial barriers that African American musicians faced despite their contributions to the music industry and American culture.