[Sample PR] Black Colormap

[story645]

I am adding a color map for shades of black
.
PR checklist:

• comma-separated file of RGB values
• Jupyter notebook with the same name as the colormap.
• The notebook contains:
  • an explanation of the colormap (what is it, why would someone use it)
  • a color swatch - we recommend using our swatch function, but it's not required
  • an example of using the colormap (can be exclusively Bokeh, Matplotlib, or Holoviews)

note: this is a sample PR, it is not intended to be merged.