This has happened two or three times over the past few days. I think the 3DS just thinks it went asleep. This has happened while playing pokemon platinum and when it happened it one time the 3DS was near a laptop and the other time it was near a ds lite. I'm not sure if that has anything to do with it. This issue quickly goes away when it happens but I'm afraid something might be wrong with the 3DS. Platinum is on an official cartridge I bought new about a decade ago, also the 3DS has not been modded what so ever.
The ribbon cable that connects to the 3ds screen could be failing at a guess, maybe in very certain positions the cable can become loose causing the black screen.
You haven’t had anything like that happen to you, have you? Also assuming you’re right is would it be easy to fix?
The only thing like that i've remotely seen is the anti piracy checks on certain DS games such as Heart Gold / Soul Silver, however as far as i'm away Platinum never had these checks, plus your using an original game anyway. As to how easy the fix is I wouldn't like to say as i've never done it. If your willing to put the time and research in I think you could probably do something like that, theirs a few videos on YouTube from a quick search I just did.