A "blazon" is an official, registered, and therefore trustworthy description of a gentleman's coat of arms, yet this reference may be figurative. "Cesario" said that he was a gentleman, and Olivia is saying that each and every one of the five things she mentioned in the previous line ("Thy tongue, thy face, thy limbs, actions and spirit") is trustworthy evidence that the young gentleman is indeed a gentleman.
Coat of Arms of England 1558-1603