If you are finding that a black liquid is coming from your pBone or pBone mini slide, this is normal and nothing to worry about. Honestly.
It might sound like something out of a horror film, but it is expected that a dark deposit will appear on your cleaning cloth and condensation will come from the water key.
This is a good thing, as it shows the stockings are doing their job smoothing down the interior of the outer slide tubes.
With use over time, this process will slowly diminish and disappear as the slide becomes quicker, quieter and smoother to run!