Because Danny is controlling Tony from the other side of the mirror, he holds the blade backwards and in his left hand (Danny is RIGHT HANDED). Repeating his 'redrum' incantation he blesses the blade with his own blood. His chant an act of telepathism - he is in another dimension bringing Halloran, he is inside the mirror world while Tony inhabits this one in his place. Unlike Jack it's easy to tell when Danny is inside the mirror world, he allows Tony to speak for him, he maintains a rigorous identity. The Hotel renames its victims, Danny uses an alternate label intentionally while alive, he blocks the Hotel's menace.