Have you notice that there is a recurring motif in Hollywood movies - the ultra-loyal, super-honest black man who serves a white hero. Typically, the white hero has a bunch of colored allies. Among these supporters, the black man is the most loyal and steadfast to the white man. An example of this is Sgt Whitaker in Final Fantasy: The Spirits Within. Sgt Ryan Whitaker (Ving Rhames) reports to Capt Gray Edwards, a very generic-looking white man. While other (non-black) members of the crew sometimes or even question the captain's judgment, Whitaker stands by him. He is always in tune to his captain's mood and shows the most empathy to the white man.
The animated feature Sinbad: Legend of the Seven Seas uses a similar formula. The character of Sinbad (the origin Sinbad is a West Asian/Middle Easterner) is drawn as a generic-looking brown-haired white guy that Euro-American audiences can identify with. He leads a crew that includes one black man, a brown-skinned Spanish-speaker, and 2 Chinese guys . The black man, Kale, is the ship's first mate. The official Sinbad the movie site says, "Of all the crew, his loyalty to Sinbad is unquestionable." Kale always supports Sinbad's decisions even if others question Sinbad.
An example of the loyal black ally is Le Chocalat (Deobia Oparei) in Moulin Rouge. The black man is always there for the white heroine, first catching her as she falls during her performance, and later saving her from the bad guy and of course, supporting her relationship with her lover.The TV series "Walker, Texas Ranger" also featured the white hero-black sidekick combo.
There is nothing wrong with an individual ultra-loyal, super-honest black character. That is much better than negative black stereotypes. But I still suspect such characters were created with the primary purpose of making white people feel safe and good about the presence of a black character in a movie. If the situation had been reversed - a black hero with the ultra-loyal white follower, I'm not sure if white audiences will find things as appealing. I can only think of one example of a TV series with a black hero-white sidekick combination - the cartoon series Static Shock.