The sign for "blood" is a combination of the signs "red" and "trickle down."
In this example, my left hand (my non-dominant hand) isn't touching my body. (And is not supposed to either -- so that's good eh?.) The handshape of the non-dominant hand can be a loose "B" or "4" hand.
Just hold your non-dominant hand in front of your chest, while you do an abbreviated version of RED with your dominant-hand index finger and smoothly transition into a "trickle down" movement over the non-dominant hand (fluttering the fingers of the dominant hand just a bit as it goes down).
BLOOD:
If someone is getting their blood drawn, you can sign that by starting with a bent-5 hand at the inside crook of the non-dominant arm. Move the hand forward and down along the arm a bit as you change the handshape into a flattened "O" shape. (Note: A bent-5-hand is like a claw-hand but the middle and small knuckles are straight. The fingers are bent at the large knuckle but not the other two.)
DRAW-OUT-BLOOD:
To sign "blood pressure," use a right "C" hand. Pat the left bicep twice as if showing the cuff used when taking someone's blood pressure.
BLOOD-PRESSURE:
Note: You may see some people combine the signs BLOOD and STRESS-(pressure) to mean "blood pressure."
Notes: