Dark Boos are stronger, purple versions of the Boo.
Dark Boos are a Boo sub-species that is much stronger than their species origin. They appear through several several games since their debut in Paper Mario: The Thousand-Year Door.
Dark Boos are purple Boo-like enemies appearing in Paper Mario: The Thousand-Year Door, Super Paper Mario, and Mario & Luigi: Partners in Time. They are known to like art. They are located in the Pit of 100 Trails, Poshley Sanctum, and Gritzy Caves.
"That's a Dark Boo. Purple just looks... so wrong. These Boos are SO creepy. Max HP is 8, Attack is 5 and Defense is 0. Its HP and Attack are high, but they're just like ordinary Boos otherwise. So, just attack it like a normal Boo. Get it before it turns invisible!"