Rashi: It must be mixed so that it shouldn’t coagulate and should still be suitable for זריקה (literally: throwing).
It is obvious that coagulated blood can be “thrown”, so why does it need to be mixed?
It is clear from this Mishna, that it is not sufficient to merely throw the blood, rather it must be “shpritzed”. Once coagulated, it can no longer be shpritzed. Therefore, it must be mixed to prevent coagulation.