Not sure if this is the place to do it but I'm going to leave my prediction here:
The one who murdered Mary in Blithe's dream/vision and the person Dahlia's messages are referring to is probably Alfred. I'm already pretty sure both are caused by the same person because of how they're both related, Alfred has already shown that he's not mentally...there and would most easily explain why Dahlia would know anything like that in the first place, not to mention how secretive and casual she is when giving us the information (It could just be how would act normally, but being that secretive points to the culprit being someone close to her, otherwise she wouldn't need to do it). There are other story-telling reasons imo that point to it being him, but I'll leave those out for now