I'm skeptical of these claims that are now presented as inherent truths. I don't think Chinese people are any more or less prejudice against black people than like every other group of non-black people in the world. My personal opinion is American companies decided Chinese people hate black people, without much real evidence, and from there it's just escalated into a type of Sinophobic "truth."
When I lived in Japan, I vibed really well with Chinese people I met and knew. Granted, they were foreign language speakers, either of Japanese, or English or both, and thus likely inclined to be more tolerant of whatever outside racial/ethnic groups. There's also the element of us all bonding as non-Japanese people living in Japan. Also while there, through a friend of a friend, I was introduced to a black American who taught English in China for a number of years, before moving to Japan to try teaching. He was in JPN for a month before moving back to China, saying he was more comfortable, made a lot more money and his overall quality of life was much better. He also stated he never wanted to return to the United States.
The NBA is also in-fukking-sanely huge over there. Chinese people hand the National Basketball Association $4 billion (and growing) a year. The argument that Chinese people are comfortable with black entertainers being on their television screens as athletes, but not as actors in Western companies' commercials or Western movies is... well, I mean, it's plausible, but it sounds more retarded than logical, to me.
For the record, I never saw us in Western product advertisements or Western store signs in Japan or South Korea (US military allies) until around 2016, near to when I left. EDIT: As other posters have demonstrated, this happens all over the globe - my position isn't to defend China, it's to point out that China is being singled out for something that's done pretty much everywhere around the globe and in countries that white Americans love.