In "The Masque of the Red Death" by Edgar Allen Poe, "blood imagery" isn't a special kind of imagery. Like any imagery, it references a particular thing: blood or symbols of blood.
Imagery is language that conjures up mental pictures and/or recollections of sound (e.g., work horses' clomping feet), taste (e.g., warm cherry pie a la mode), what something feels like ( e.g. velvet), warmth/cold (e.g., cuddly down blanket; icicles outside the frosted window pane), smell (e.g., cinnamon rolls baking), and movement (e.g., careening of a ship tossed on rough waves).
The language of imagery can consist of literal descriptions (e.g., fields of waving wheat in the morning sun seen out the window beyond gently blowing white lace curtain), simile (e.g., biting into the chocolate truffle was like biting into a chocolate marshmallow cloud), metaphor (e.g., she was a rock of determination), or allusion (e.g., as impetuous as Icarus).
The imagery relating to blood in "The Masque of the Red Death" is either literal or symbolic in the form of metaphor. The earliest presence of imagery is "blood was its Avatar and its seal--the redness and the horror of blood." This image mixes metaphor "was its Avatar and seal" with literal "redness and the horror of blood". Poe is describing through imagery the aspect of the red death plague.
The second instance of imagery is "scarlet stains upon the body...." This instance is a metaphor. Poe has established what happens when the disease strikes, now he metaphorically calls the symptom of bleeding pores a "scarlet stain." The third instance is another metaphor. Poe says that at the sealed off abbey "there was Beauty, and there was wine." Wine was frequently used a a metaphor for blood: They are both red and they are both said to course through the body. This metaphor severs as foreshadowing, a clue to his surprise ending.
The fourth instance is a little more complicated metaphor. Poe says that outside the corridor window of each of the seven rooms there stood a tripod (three-armed) brazier (holder of coal or other fuel for heat and light) that projected it rays through the tinted glass of the windows. In this instance, Poe is using "brazier of fire" as a metaphor for blood. Again, both are red and, as blood transfuses the body, the light of the fire transfuses the rooms.
There are many more, "fire-light" on "countenances" (i.e., bleeding pore metaphor), "bold and fiery;" and now that you are started, I'm sure you can find your way to others.