Zevachim 78A

רבי יהודה אומר: אין דם מבטל דם.

R' Yehuda says: Blood does not nullify blood. Therefore, the priest presents the blood of the mixture on the altar.

נתערב בדם פסולין ⁠– ישפך לאמה. בדם התמצית ⁠– ישפך לאמה; רבי אליעזר מכשיר. אם לא נמלך ונתן ⁠– כשר.

If blood fit for presentation was mixed with the blood of unfit offerings, there is no remedy. Therefore, the entire mixture should be poured into the drain running through the Temple courtyard. Likewise, if blood fit for presentation was mixed with blood of exudate, i.e., that exudes from the neck after the initial spurt following its slaughter concludes, which is unfit for presentation, the entire mixture should be poured into the Temple courtyard drain. R' Eliezer deems this mixture fit for presentation. Even according to the first tanna, if the priest did not consult the authorities and placed the blood on the altar, the offering is fit.

גמ׳ אמר רבי חייא בר אבא אמר רבי יוחנן: לא שנו אלא שנפלו מים לתוך דם, אבל נפל דם לתוך מים ⁠– ראשון ראשון בטל.

Talmud: The Mishnah teaches that in a case where water became mixed with the blood of an offering, if the mixture has the appearance of blood it is fit, despite the fact that there is more water than blood. Concerning this R' Ḥiyya bar Abba says that R' Yoḥanan says: They taught this halakha only in a case where the water fell into the blood. But in a case where the blood fell into the water, the first drop of blood, and then the next first drop of blood, is nullified in the water, i.e., each drop is nullified in turn. Consequently, the mixture is unfit for presentation, regardless of whether it has the appearance of blood.

אמר רב פפא: ולענין כיסוי אינו כן, לפי שאין דחוי במצות.

Rav Pappa says: But with regard to the mitzva of covering the blood of birds or undomesticated animals that are slaughtered, it is not so. In this case, even if the blood fell into water, the mitzva of covering applies to it, provided that the mixture has the appearance of blood. The blood is not nullified by the water because there is no permanent rejection with regard to mitzvot, i.e., its nullification was merely temporary, but once there is enough blood in the water, it reassumes its status of blood.