A Blood Seal is a badge of office given to certain high-ranking operatives of the Marshal of Faerie; it gives the Bearer (the title of one who holds such a badge) blanket authority to do anything and everything necessary to uphold the Crowns.
It is comprised of a magicked gem set within an enameled border. As seen in "The Thin Line," drops of blood from the would-be Bearer are shed upon the gem, tying the Seal to the bearer (and also ensuring its genuineness). When activated, it produces a distinctive flash of colour and a three-note chime (which vary from Bearer to Bearer), and an image of the Crown is shown.
While not stated, it is implied that the Blood Seal expires either upon the death of the Bearer, or upon the death of the High-King and Emperor under whose authority it was granted.
Westersloe Winterbough V, Meadow Grainmaster, "X" and "Y" are the four principal Bearers during the events of The Winterbough Saga. Winterbough observes that the sight of the Blood Seal usually terrifies any to whom it is shown, as it usually portends great trouble.