Chullin, Chapter 6, Mishnah 6. This chapter is devoted to kisuy hadam, the mitzvah of covering the blood of a slaughtered chayah or of, and here the Mishnah turns to a case where the blood never reached the earth at all.
Begin with the language of the Torah itself: "veshafach et damo vechisahu be'afar" - he shall pour out its blood and cover it with earth. The Sages understood from this wording that the mitzvah is not simply a matter of throwing dirt over blood that has already settled on the ground. The earth is prepared in advance, so that the blood is poured onto dirt, and then more dirt is placed over it. In other words, kisuy hadam involves two layers of earth: one beneath the blood and one above it. The blood should not be resting directly on the bare ground with only a covering on top.
Blood that missed the earth
Given that requirement, our Mishnah asks what happens when the blood landed on a surface that is not fit for kisuy hadam at all. The Mishnah rules: "dam hanitaz veshe'al hasakin, chayav lechasot" - blood that spurted out and struck a wall, or blood that remained on the knife after the shechitah, must still be covered.
The remedy is straightforward. One scrapes off the congealed blood, whether it has dried on the plaster or remains on the blade, gathers it, sets it down upon dirt that has been readied beforehand, and then spreads further dirt over it. The blood once again sits between two layers of earth, one underneath and one on top, precisely as the verse describes.
Rabbi Yehudah's qualification
Rabbi Yehudah then narrows the obligation. "Amar Rabbi Yehudah, eimatai" - he asks in which circumstance this ruling holds, and answers: "bizman she'ein sham dam" - when there is no blood to be found there, "ela hu" - apart from this blood itself. If the drops splashed onto the plaster or left clinging to the blade are all the blood one has, they must be collected and used for the mitzvah.
"Aval yesh sham dam" - but if blood is present there, "shelo hu" - blood other than this, then "patur milechasot" - one is released from covering it. The mitzvah is carried out with the blood that reached the ground, and the residue that remains on the utensil or on the plaster need not be pursued.
The principle underlying Rabbi Yehudah's ruling is that kisuy hadam does not demand that every last drop of the animal's blood be covered; covering part of the blood satisfies the obligation. Therefore, if enough blood fell onto the earth to fulfill the mitzvah, the blood clinging to the wall or the knife may be left alone. It is only when nothing else is available that one must go back and retrieve it.