I explicitly stated and used the term Dark Swords when speaking about Dark Knight signature weapons. What I argued against was that the specific Dark Swords used by Cecil were katana as was stated by the poster I initially responded to. I stated that based on Amano's artwork which was the best visual representation of Cecil at the time the game was released, the swords that he used were not katana and had a more Western design. As you stated, semantically Dark Swords could be anything, but the ones that were being spoken of were not katana.
The fact that their Dark Swords were darkness aspected as a game mechanic as well as the whole concept of a Dark Swordsman wielding a special sword imbued with dark power which is sort of the basis for Dark Knight.
You really should not use anime as your reference for what things mean in Japanese, the translations into English often take a great deal of "artistic license" and often alter the meanings of things, misinterpret them or have trouble conveying the implications of words and ideas.
"Ma" in this context most certainly implies darkness in the sense of dark equating to evil/negative. The character for "ma" conveys a dark/evil spirit or force, often associated with curses in the way that a person or object can be beset or imbued with a dark spirit or force. It is not conceptually synonymous with magic as you state.
Translating "Magun" to "Demon Gun" is sort of correct but not exactly as it is really more like "Dark spirit gun" as far as a direct translation, but I would guess that they were trying to imply something along the line of "gun with ancient forbidden power", still conveying the implied dark/evil aspect inherent to "ma" but in a "cool" sense. Seeing as how "Magun" is a made up word for an anime, using it or its questionable subtitle translation as an arguing point on what the understood meaning of "ma" is is probably not the best approach.
"Mahou" can be flavored good or evil because just like its closest English translation, magic or mystical/supernatural abilities, it is general and does not have an implied good/bad/light/dark affinity like "ma" does. Saying that it can easily be taken as "evil force" or "demon magic" is a big stretch.
At this point I'm done, there is really nothing else to say on this matter and there is no point in continuing this tangential debate.
I will however leave you and others that consider themselves Japanese subject matter experts because they watch anime, play jrpgs or eat Pocky a little advice. You're not. Don't get me wrong, I think it's great you appreciate aspects of the culture, but don't put on airs because of it. Unless you are a native speaker, have lived a while in Japan or at least have a native speaker to corroborate with you, I would avoid making yourself out to be an authority. I mean the last thing you would want is to end up telling a person of Japanese heritage what is or isn't Japanese, right?