I was reworking my Giant Robot magazine and the Gloomy Bear spreads, and I was getting sick of producing blood splatters with the pen tool, so I figured out a new way to do it. You can produce lots of them, with pseudo-random variations, in a few seconds each.
Step 1: Select the star tool. Option+Click somewhere on the artboard and give the star some properties. You can copy mine.
Step 2: Use the roughen Effect. Turn on the live preview with the checkbox, and tweak the values til you're satisfied.
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Step 3: Make a few additional smaller splatters.
Step 4: If you need to, you can Expand Appearance, which will make it a regular path, rather than a star shape with Roughen applied to it.