This definitely does vary by carrier.
Basically, you were supposed to follow a specific step to authorize prior, and did not. Some insurances will make occasional exceptions, particularly if you can prove why this was unable to be done prior and the medical necessity. If it was just sort of "oops, we forgot.", some carriers will not pay regardless of how medically necessary or how many appeal letters you write.
The BCBS in my state is not Anthem BCBS, so I do not have any experience with them.
Good luck!