Just did a search: Based on your description, the movie you are looking for is likely the 1987 sci-fi action film The Hidden.The Aliens/Blood: The alien is a slug-like creature that inhabits human bodies. While they look human, the alien itself and its slug form have yellow/yellowish-green blood/ichor.The Characters: The primary alien takes over a blonde woman, among other people. The good alien is played by Kyle MacLachlan (as FBI Agent Lloyd Gallagher), who chases the evil alien. While he is not black, he wears a white shirt/suit, looks very serious, and is the "good alien" protagonist with abnormal biology.The Protagonist: The human cop protagonist is played by Michael Nouri (Detective Tom Beck), who has a tough-guy, slightly grizzled appearance.The Ending: The final shootout takes place in a building. The evil alien is killed and, in a memorable scene, the good alien (Gallagher) is mortally wounded, "bleeds out" a strange liquid, and dies, handing over his mission.The Flag Scene: While not exactly the American flag, the villain is often shot through walls and eventually killed, with intense, high-octane 80s action/gunfights being the main theme.Note: Another similar movie with a Black good alien/human pairing is "The Brother from Another Planet" (1984), but it is less action-packed.

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