Role in the Fanfic
While engaging in sexual activities with Draco, Ebony falls asleep (subtly suggesting Draco wasn't all that good). She then experiences a dream where she sees "a black guy" shooting "two goffik men with long black hair." They plead with him to spare their lives, but he continues to shoot them. At the end of the dream, he is said to have "ran away in a red car."
The two men are eventually revealed to have been Lucian and Serious, and the location of the dream was in the well known city of Longdon.
Reader Reaction and Controversy
Tara Gilesbie had already received flack from readers for her use of slurs for homosexuals, and it appears that people weren't very happy with her description of "a black guy" shooting up two characters gangster-style. Given no other characters in My Immortal are black (or at least identified as such), it would indeed seem odd that the sole black character to make an appearance comes across as a bit of a stereotype.
Tara dismissed any accusations of racism on her part. She later wrote in her infamous author's notes: "oh y and I wuznt beng rasist ok!11" Many are unsure if it would be possible for Tara to hate anything black.
The black guy is never identified, let alone mentioned again, for the rest of the fanfic. Although Vampire, Draco, and Ebony convince Dumblydore to find Lucian and Serious, no one ever looks for nor attempts to discover the black guy's identity. When Vampire is first told by Ebony of her vision and asks, “What fucking dick did that!”, Ebony responds that she doesn't know. When Serious and Lucian recover from their wounds (which takes practically one chapter), they never clue anyone in on what the black guy looked like. In fact, no one ever shows any interest in finding the black guy, and he is quickly forgotten about. As such, his true name and identity remain a mystery.
One possible identity is Voldemort himself, but in disguise. The Bark Lord had previously given Ebony a gun, showing his affinity for the weapons. His antics with the MCR concert further show that he is a master of disguise, and capable of appearing as other people convincingly. It could be that Voldemort disguised himself as a black guy to shoot Serious and Lucian, though why he would have done so is unclear.
Another possible identity is that the black guy was merely an unknown Death Dealer, under the employ of Voldemort but not known yet before then. The Death Dealers are seen using guns, as witnessed by Snaketail's gunfight with Vampire and Ebony.
Yet another theory (though related to the previous one) suggests that the black guy may be a canon wreckage of Kingsley Shacklebolt. Though Kingsley was not a villain in the original canon, Tara's treatment of Lupin suggests she really doesn't care who she turns into a servant of Voldemort (certainly not anymore than she does moving people between houses). That she would turn Kingsley from an educated, well respected auror into a gun toting black stereotype is also not surprising, given she turned Lupin from a friendly, warmhearted teacher into a "necphilak."
One interesting theory is that the black guy is actually a horcrux of Voldemort. The entire episode with Ebony's vision of Lucian and Serious being attacked and then contacting Dumblydore about it is similar to an incident in the original canon where Harry sees a vision of another character being attacked by a creature later revealed to be a horcrux of Voldemort's. Though horcruxes are never utilized in My Immortal's storyline, Tara is well aware that they exist, as shown by her author's notes near the end of the story. If she truly did base the incident with the black guy, Lucian, and Serious after the scene from the original book/movie, then that would suggest the black guy was one of Voldemort's horcruxes, and hence played a much more important role to the story than previously imagined.
A more natural explanation suggests that the black guy could simply be a muggle thug or gangster that, for some random reason, decided to shoot Lucian and Serious. There is no immediate connection made between the shooting and anything else that happens in the story; therefore for all we know Lucian and Serious could have been simple victims of street crime.
Most readers simply assume Tara wrote something she thought was funny, and then quickly forgot about it.