Does the blood on the kitchen knife not match that on the accused's clothes? That's exculpatory evidence: anything that clears someone or something of guilt or blame is exculpatory.
Exculpatory comes from the Latin word exculpat, meaning "freed from blame." The verb exculpate means to free from guilt or blame. Both words are used most often in a legal or technical sense rather than in everyday conversation — unless of course you're trying to show off.