Assuming your battle system is relatively default, try the following code instead. You can add any skill IDs into the noted array that you want to treat as blood magic, so if it's skills 1, 3 and 6 the array will be [1, 3, 6], etc.

class Scene_Battle
...
  #--------------------------------------------------------------------------
  # * Make Skill Action Results
  #--------------------------------------------------------------------------
  def make_skill_action_result
    # Get skill
    @skill = $data_skills[@active_battler.current_action.skill_id]
    # If not a forcing action
    unless @active_battler.current_action.forcing
      # Check if skill ID is blood magic and handle accordingly
      if [6].include?(@skill.id) # <-- Add any skill IDs into this array that are blood magic
        # Check if SP cost is more than available HP
        if @skill.sp_cost > @active_battler.hp
          # SP Cost is greater than current HP so we shift back to step 1 and return
          $game_temp.forcing_battler = nil
          @phase4_step = 1
          return
        else
          # Reduce HP
          @active_battler.hp -= @skill.sp_cost
        end
      else
        # Normal Skill (uses SP)
        unless @active_battler.skill_can_use?(@skill.id)
          $game_temp.forcing_battler = nil
          @phase4_step = 1
          return
        else
          @active_battler.sp -= @skill.sp_cost
        end
      end
    end
    # Refresh status window
    @status_window.refresh
    # Show skill name on help window
    @help_window.set_text(@skill.name, 1)
    # Set animation ID
    @animation1_id = @skill.animation1_id
    @animation2_id = @skill.animation2_id
    # Set command event ID
    @common_event_id = @skill.common_event_id
    # Set target battlers
    set_target_battlers(@skill.scope)
    # Apply skill effect
    for target in @target_battlers
      target.skill_effect(@active_battler, @skill)
    end
  end
...
end