The blood should always be darker than whatever color it's on, so when in doubt just use saturated red and set it to 'multiply'
Thicker blood = darker (+highlights), thinner blood = less opacity; yk it works since it'll make any color look like blood
Though I usually just use bright red by default and make it darker whenever it looks off, and for the smears/bloody fabric I just eyedrop the color under it, set the hue closer to red and make it more saturated & dark...
I find it easier for simple drawings cause I got used to doing it before I realized multiply basically does it for you :P
So I like to start with bright red, then add dark red in the center/where the actual wound is, and add smears in random places
If you're like me and try to keep unique colors to a minimum, you'll have to draw the smears yourself, then add a bit of bright red at the edges/concentrated areas... for everyone else, you can just use a blending tool/soft brush
Also if the blood has been there for a while, you can make the blood/smears more brown/orange, respectively, for a more gross & realistic effect
As for the actual shape, I like to make the brush bigger (with pen pressure on) since it's harder to control and will make irregular, blobby shapes easier
EDIT: am no longer able to use pen pressure, so i kinda just have to deal with it :P
When drawing, try not to overthink it :]
I've seen people do that thing where they draw a rough shape then erase random parts of it; I never liked that method since I think it looks unrealistic, unless the surface is very waterproof...
That's all I got for now, I'll add stuff later if I think of it
-friend that proof-read this before i uploaded it