Biography
In many series, there will be a character who is much larger and more intimidating than most everyone else, very strong, often quite quiet, and very often this character will be black.
Sometimes they can be the Token Minority, sometimes they're from a Proud Warrior Race, and sometimes they're a Gentle Giant. But the fact remains that when you get right down to it, they're a scary black man. Not necessarily evil, just... scary.
Authors will often claim this is not actually done in a racist way (not intentionally, anyway), as most anybody who is so much larger than the other characters will usually be equally scary (unless they're also outgoing or jolly). However, perhaps because of a lack of very large Asians who aren't sumo or very large white people who aren't professional wrestlers in a cameo role, they are often black. (This is the same reason that a Huge Schoolgirl usually isn't mean, either.) However, American media depictions cannot escape the legacy of the Big Black Buck (as featured in such influential films as D. W. Griffith's pro-Klan propaganda film The Birth of a Nation), the savage (noble or not), or modern thug culture. Amongst other reasons, when much of the creating population and the consuming population is not of African descent, then the Black Guy becomes Other, and as such, easier to position as an imposing, scary force.
The black characters in anime are usually some variation on this.
See also Scary Minority Suspect. Often Crosses over with Gentle Giant, Genius Bruiser, Proud Warrior Race Guy, and Token Minority, as noted both above and in the examples below. Compare "Angry Black Man" Stereotype.
Note that the character doesn't necessarily have to be of African descent, just large, imposing, brown/dark-skinned, and have a tendency to make people wet themselves with a single glare. If a Black male character has other characters fearing him after he performs a certain action he is not a Scary Black Man, a Scary Black Man has people fearing him because of his intimidating appearance. Despite some of the Unfortunate Implications associated with this Trope, some of these characters become popular because of how badass they are. The obvious subversion is to make this character not nearly as scary personality-wise as their imposing first appearances might otherwise suggest. Another subversion can be to have the character only act this way in certain specific situations — a man might be a loving father and an otherwise affable and easygoing person... who immediately turns into an enraged Papa Wolf whenever his loved ones are threatened.
Despite the trope name, female examples do exist. But since female characters are rarely portrayed as fighters or overtly threatening in the same way that male ones are, Scary Black Women remain a distinct rarity, and are more commonly portrayed as sassy black women instead. Contrast Intimidating White Presence.