The Sprites accidentally released the shadows
- At the start of the special, the girls mention that Astra has to keep the stars lit or else shadows will take over Lite-Topia. In the flashback, the girls found their light wands in a crystal cavern, hanging from the ceiling. As the shadows taking over apparently wasn't a problem before the girls got their wands (Lite-Topia is shown as pretty colorful even before the girls got their wands), one can assume that the wands were in the cave because they were sealing the shadows away.
Bleak's wand was made as last resort to prevent shadows from acquiring the colors of Lite-Topia
- Assuming the shadows are attracted by anything with colors, Bleak taking colors away would surely render Lite-Topia less lively but might also spare whatever she discolored from the shadows until another Sprite colors it back.
Bleak's wand doesn't just take color away, it lightens up things
- Notice that in the opening, when her friends make the show's title with their wands, she's the one to make the letters brighter and more visible. Bleak might have, as ironic as it'd be, a light power in addition to removing the colors, which, combined with the top WMG, would explain why her wand would've also served in containing the shadows.
- In addition to that, albeit it's hard to notice, she's also glowing when she and the girls are in the cave after she comes looking for them. If her power was simply of a dark nature, she would not glow like the rest of them.