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This effect can be achieved by drawing the blood in two layers. The bottom layer is the black border and the top layer is the red blood. Note that actual "layers" are not required, as long as the drops can draw in two phases, all black, followed by all red. Each droplet draws its shape as a black sprite. Then after all black shapes are drawn, they each draw ...
If you look closer - it is not border of(each) blood drops - it is a border of all blood-red area. And I think the border was added actually after rendering blood to look cartoonish. This could be post processing effect just as well as shader. It should not be difficult to implement edge-detection algorithm (plus you know the color you are looking for).
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