Wed Sotah 16

R' Yismael holds you need to put the bird's blood into a Revios, since you need to recognize the blood in the water.

Tosfos explains: it doesn't mean to recognize that the water is red, since even a much smaller amount is recognizable. However, this would be Batul (like the Rabanan that hold that something is Batul in its own kind.) So here we need enough that the thickness of the blood is recognizable. By that it wouldn't be Batul.

R' Akiva Eiger asks: if so, the Rabanan that argue on R' Yishmael (that you place it in more water and the thickness is not recognized) the blood is Batul? Furthermore, since the water is a different kind with the blood, the blood is not Batul and we would say if anyone drinks some of the water he's Chayiv for drinking blood.