Where the blood was ineligible for covering, and then became eligible: Succah 33a
The blood of a koy: Beitzah 8a-b
A mixture of blood from animals that require and don't require covering: Beitzah 8b
Concern that if one covers a Koy's blood, people will assume its fat is permitted, since one may eat the fats of those animals for which blood-covering is a requirement: Beitzah 8b