I would pick a different name.
Im sorry. I think Blake is often associated with being a boy's name, so having a so-called boy's name plus the initials BJ might not be fun for her growing up.
I had a friend who's initials were B.O. and she got teased.
My mom almost named me Vanessa, but our last name started with D and she didn't want my initials to be V.D. (which as it turns out, isnt an abbreviation that is used or well known anymore.) There's a chance maybe the next generation won't know what a BJ is or may have a different word for it, but I doubt it.
Also, I dont think it matter when high school starts; at some age kids will learn what a BJ is