Commentary for Avodah Zarah 64:11

The mishnah taught that it is prohibited to derive benefit from skins pierced at the heart. It is assumed that such skins were part of idolatrous practice. This section defines precisely what this heart-pierced skin is.
The presence of the blood is necessary to determine whether the hole was made while the animal was alive. If the hole was made after its death, then there should be no blood and it is permitted. Unless the skin was salted in which case we can assume that the salt removed the blood.