I've been fascinated with these two ghosts for a bit, and though Pottermore won't properly touch on them for a while, I thought I'd give a shot at a bit of fannon before it shoots the theory down.
So as for the Bloody Baron and the Grey Lady, we know this much. The Baron loved the Grey Lady, Helena Ravenclaw, and it was fairly evident that she didn't love her. After tracking her down to the forests in Albania, he wanted to bring her back (at the behest of Rowena Ravenclaw) and Helena stated that, "When I refused to return with him, he became violent. The baron was always a hot-tempered man. Furious at my refusal, jealous of my freedom, he stabbed me." Now this is what I realized is so odd.
Why would a man, an accomplished wizard recognized by Rowena Ravenclaw as being capable to bring her daughter back, use a knife? Why wouldn't he use his wand? Seems quite easy: use the wand, Petrificus Totalus, grab her and apparate off. So why on earth would he stab her? Even if you wanted to say that he was hot-tempered, that he was furious at her, why wouldn't he just hex her? I thought about this for a while and it hit me. Wizards and witches only used wands against other wizards and witches, but apparently not against muggles according to information on the Sword of Gryffindor in Pottermore.
As such, I have three theories as to what actually happened
- Helena was considered to be weaker in some manner (lack of offensive spells/gender). This is something that I would think fits a LOT of what she says without saying it. Helena wanted to show up her mother, she took the Diadem because she was jealous of her, and wanted to be smarter and cleverer than her mother. It could be that due to a bit of that knowledge, the Baron would have used his knife rather than his wand to coerce her, knowing that she didn't have a supply of offensive spells to hit him with. In addition, it could be that he considered her a non-threat, as she was a woman, also encouraging the use of the knife rather than the wand as the wand was reserved and not to be used in fights against muggles as it was "unsporting"
- The Baron dueled Helena, and was disarmed. For those wanting to say that Helena was a good witch, this makes a bit more sense. Helena disarms the Baron and turns her back on him, thinking that to be the end of it, telling him she does not love him in the slightest, and that she'd rather wander than be tied to a miserable wretch like him. He gets mad and stabs her while her back is turned. This is definitely something I can see matching what she said roughly, though I would still wonder why she'd leave the duel out.
- She stabbed herself with the Baron's knife. This is honestly what I think is the LEAST likely of the three. To resist him taking her in to her mother, she summons the Baron's knife and stabs herself with it. The Baron is still wrought with guilt and stabs himself to be with her in a moment of passion.
The Bloody Baron and Grey Lady's story is an odd one, and one I'd definitely to see with a mini-story, akin to what we saw done with the Tales of Beadle the Bard. Even a bit on pottermore clarifying it would make a good deal of sense, though to be honest, this is a story that I think could be given it's own plot, if it follows them from school to that moment where she dies, and most of all, I hope that the use of the knife will be explained a bit more, though it may be one of those weird moments that JKR just forgot to write about in detail.