I don't think it's Tigerstar's betrayal alone that made Bluestar go into depression, she's stronger then that. It was build up of many things over the years; Moonflower's death, Snowfur's death, Thistleclaw's rivalry and hatred, Whitekit having to become motherless, Mosskit's death, giving away the rest of her kits, Redtail's death, Lionheart's death and finally Tigerstar's betrayal, which just happen to be the final nail in the coffin.
Also I don't Bluestar ever had feelings for Tigerstar. First, she already had a mate in Oakheart and being Thistleclaw's apprentice's didn't help. Not to mention, Tigerstar's più ambitious and battle hunger nature clashes with Bluestar's più calm approach.