It's not uncommon for such a twist in BL manga. The boy next door could totally be a vampire if the story is set up that way to add excitement and mystery.
The settings in BL vampire novels are usually quite dark and gothic. Think of old castles or shadowy cities. This setting helps to create an atmosphere that is both spooky and romantic for the BL relationship to unfold. For example, in many such novels, the vampire's lair becomes a place of both fear and a sort of strange intimacy for the lovers.
They usually have a focus on romantic relationships between male characters and incorporate elements of vampire lore like fangs, blood, and supernatural powers.
One common theme is the power imbalance. Since vampires are often depicted as more powerful than humans, in a BL context, this can create an interesting dynamic between the two male characters. Another theme is the struggle with identity. The vampire character may be struggling with his vampiric nature while also dealing with his feelings for the other male character.