Significance of The Blue Chalk

When Father Callahan and Mark Petrie encounter Barlow at Mark's house, Barlow grabs Mark and threatens to kill him unless Father Callahan drops his crucifix. While Barlow is holding Mark, Mark notices small smears of blue chalk on two of Barlow's fingers. Later, when Mark, Jimmy, Matt, and Ben are trying to figure out where Barlow is hiding, Mark mentions the blue chalk. At first, they think that the blue chalk is school chalk and that Barlow is hiding in a school. After Jimmy sees a toy pool table in the home of a vampire family, however, he realizes that the blue chalk is probably from a pool table. Jimmy and Mark remember that there is a pool table in the cellar of Eva's boardinghouse and realize that Barlow is hiding there. The blue chalk is the clue that allows Jimmy and Mark to find Barlow and allows Ben to destroy Barlow in the end.