Keep in mind that she will likely either become Blue or Belle in day to day life. There seems to be an abundance of little Belle/Bella/Ella/Elle’s (though I still like all of these).
So to me the question is are you most attached to the Blue part of the name? Because you could have an Isabelle, Annabelle etc and still reach the same day to day name.
If it’s nature/flower names you like, already some good suggestions, I think Fleur works too.
If it’s the slightly alternative sound of Blue you like, consider Winter, Alma/Alba, Taiya, Tansy.