This blood was Alani's, if you hadn't gathered, she was sent to this room to get punished. Alani has been sent to this room almost every single day since she was Two years old, her Uncle wanted her to get immune to pain, so he beat her up. Mr Riddle dragged the wooden chair aside and Alani was hung up by her wrists, her feet were dangling an inch above the floor. Mr Riddle wasn't nearly as nice as Filch when he hurt her, for example he cut off her legs and arm, her legs were cut off when she was Seven, her arm was cut off when she was Eight, the damage he did to her depended on how angry he was. If Artemas or Mattheo misbehaved or broke stuff she took the fall for them, sometimes they'd take her place and admit that they did it, that's why they had a few scars on their bodies. But Alani managed to take the fall most of the time, to protect her Brothers innocence.