The plague of the 1300s was called the black death because the disease turned the patients'lungs black. the first cases happened near the black sea. the bodies of victims turned black after death. the illness created black spots on the body.

as for symbolic actions, the narrator's tearing down the wallpaper in an attempt to find the "woman" in the wallpaper represents her struggle to retain or regain her sanity.   the wallpaper has been part of her confinement and by her tearing it down, she is freeing herself from that confinement.