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 a red shape which is smaller around the edges by 1-2 pixels. If the shapes are circles, then this means scaling by 1-2 pixels in width and height. For non-circular shapes, you can just have two sprites, one smaller than the other. This can be achieved via the "Inset" command in GIMP or Photoshop.
This achieves the desired effect because every part of the black shape is covered up by the red shapes, except around the edges where the red shapes aren't quite big enough. In interior regions where there are many blood particles, the black is hidden entirely.
No shaders are necessary, although they would be if you want it to be lit properly as if the blood was 3 dimensional.