A. I not only knew her, but was her best friend.
B. Not only did I know her, but I was her best friend.
That is what I would use in AE. In sentence B the subject "I" is part of the "not only" clause, so must be repeated in the "but" clause. In sentence A the subject is outside the "not only" clause, and can be used for both parts.
Not only was it blue, it was bright blue! It was not just blue, but bright blue!