Rashi on Leviticus 14:28/רש"י על ויקרא י״ד:כ״ח

1על מקום דם האשם UPON THE PLACE OF THE BLOOD OF THE GUILT-OFFERING — even though the blood has been wiped off; this teaches that it is not the presence of the blood that brings about the validity of this rite but it is the place (the fact that the oil has been placed on the proper spot) that brings it about (Sifra, Metzora, Chapter 3 10; Menachot 10a).