Hullin, Chapter Six, Mishnah Six
Our mishnah deals with whether all blood must be covered up or just some of it.
1) The blood which spurted out and that which is upon the knife must also be covered up.
2) Rabbi Judah says: when is this the case? When there is no other blood but that; but when there is other blood besides this, it need not be covered up.
Section one: Blood which is spurted out refers to blood that is found at some distance from the place where the animal was slaughtered. Such blood and blood that is found on the knife must also be covered up.
Section two: Rabbi Judah limits the above law to cases where this is the only blood. If this is the only blood with which to fulfill the commandment of covering the blood, then this blood must be used. But if there is other blood, at the place of slaughter, then he should use that blood and the blood on the knife or found elsewhere need not be covered.
In mishnayot like this one, it is hard to tell whether there is a debate or whether Rabbi Judah is simply explaining the first clause. If there is a debate, then we might understand that there are two concepts with regard to covering the blood. According to the first opinion, all blood must be covered, regardless of where it is. The blood, the life force of the animal, cannot be left in the open. Perhaps doing so would involve some sort of danger.
In contrast, Rabbi Judah says that there is a mitzvah to cover the blood, but not that all blood must be covered. The purpose of covering the blood is, perhaps, to atone for the killing of the animal, the taking of a life. In that case, someone must cover some of the blood as a symbolic act, but he need not cover all of it.