Yeah , lovebaltor is right it not ever season 1 it's two . episode 23 is when she is captured The time for truth ( Darkness and Light) , in episode 24 is when she becames dark , Darkar's Prisoner ( Desperately Seeking Bloom) in episode 25 is when she is entirly dark ( i mean all the epsiode dark bloom ) , Face to Face with the Enemy ( Storming Shadowhaunt ) . In epside 26 is when they resuce her, The Phoenix Revealed ( The Ultimate Power Couple ) .
And their is epsisode 19 she is dark when she steals the codex from alfea but he has only her eyes not costme dark and she turn good again , Spy in the Shadows ( Shadows in Bloom )
It's not even in season one :P Bloom turns evil in season two when Avalon preforms a curse on her. (To aid Darkar) The episodes in which she appears in are: Darkar's Prisoner (Desperately Seeking Bloom) and The Phoenix Revealed (The Ultimate Power Couple). She makes other small appearances in other episodes, but those are the main two in which she does.