Spooky as
Dark Shadows is, we've counted only seven instances of "boo."
The first is an excellent way to handle a haunted portrait, from episode 2, via DarkShadowsForever.
The second is from episode 126, the third from 169, and the fourth (a frenzy of multiple boos) from 252.
The sixth reveals that boos are pearl-shaped tones, from 313.
The seventh (saying goodnight to a ghost by the side of the bed) is from 506.