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,

Reuven Weiner