In the 14th century, Europe was ravaged by a deadly plague which came to be known as the Black Death. This disease spread rapidly across the continent, wiping out somewhere in the region of sixty percent of the entire population, a truly staggering amount. If anything, the Red Death in Poe's story is even more deadly. It's devastated the kingdom, leaving misery, suffering, and painful death in its inexorable wake.
Blood was its Avatar and its seal—the madness and the horror of blood.
The red of the Red Death is the red of blood. If you're unlucky enough to be stricken with this foul pestilence, then you can expect to bleed profusely from every pore. Once the plague grabs you and holds you in its vice-like grip, then your entire body will be covered in hideous patches of blood. Worse still, the blood will cover your whole face, ensuring that you'll receive no help or sympathy from anyone, as they'll be too horrified to intervene.