Fresco said: What's that black stuff? It worries me.
It's probably not blood, given that it's still black even when in slightly brighter lighting. It could be pussy juices, though the largely solid color regardless of lighting could bely that.
If you've ever rubbed water into a dark hardwood, you'll have noticed that it temporarily becomes even darker. With a very thin stream, it wouldn't be surprising for the contrast to make it appear almost black.