E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To blazon is to announce with a trumpet, hence the Ghost in Hamlet says, But this eternal blazon must not be to ears of flesh and blood, i.e. this babbling about eternal things, or things of the other world, must not be made to persons still in the flesh. Knights were wont to be announced by the blast of a trumpet on their entrance into the lists; the flourish was answered by the heralds, who described aloud the arms and devices borne by the knight; hence, to blazon came to signify to describe the charges borne; and blazonry is the science of describing or deciphering arms. (German, blasen, to blow.)