He was proven that he likes cranberry-apple juice in the episode 'Bye Bye Nerdy'. He asks another friend, Bulby, to bring him some juice then complains about it not being "Cran-apple". Although, just like Bloo, he drinks it anyway.
And I think that Bloo liking celery was just used to show how Bloo will
always find something to complain about (one of my favorite aspects of his personality, too
). It probably also helps diversify his tastes and make him seem more real, much like his countless allergies. After all, we all like
something other people would find gross.