My 2 cents on the translation of the afterword. IMO, 黒歴史 (Dark History in English) means alternative continuities or "what if". It comes from Turn A Gundam back in 2000. It does not have a negative connotation.
Ref : https://ja.wikipedia.org/wiki/%E9%BB%92%E6%AD%B4%E5%8F%B2