If blood came out of the animal of the first born why is it exempt from bechoros?
We are discussing blood that comes out of the mother animal. Rebbi Eliezer ben Yakov refers to a Chararah of blood from a Behemah Gasah, which Rabeinu Gershom (19b) explains is a hard chunk like a biscuit ("Chararah"). This is obviously more than thin blood and shows evidence of a fetus.
Before, when Tinuf is mentioned, it is also more than simple blood. The Gemara (21b) says that it is checked by a wise shepherd who recognizes the aborted fetus.
All the best,