Painting your pet's portrait is a fun but challenging project. This is especially true if you cat, dog (horse or rabbit etc) has solid black fur or hair.
What appears to be a solid black color is actually an illusion.
Look carefully at your subject in bright sunlight and you may see undertones of blue, purple or brown. This is especially true in afternoon light.
If you gently part the animal's fur and look at their underlying skin tones, you will see every shade from pale pink to silver-gray and white.
This under-tone plus the "temperature" (warmness or coolness of lighting) will contribute to what appears to be black fur.
Now we've done our field research; how does this information translate into an attractive, enjoyable painting?
Before you pick up pure black paint and use it to define the animal's coat, take a small amount of blue, purple or brown - based on your previous observation - and apply that color as an underpainting.
Allow the underpainting to dry completely. Work on another part of the painting, such as background.
When underpainting is dry to the touch, go over it with black paint MIXED with a bit of the same color used in underpainting.
Don't try to totally cover the underpainting with the black mixture! Let some of the underpainting show through!
This method will work equally well with watercolor pigment or acrylic paint. (I've never tried it with oil paints, so I can't recomend that)
Just remember that a solid black pet is truly not solid black but a combination of undertones, fur and the prevailing light source.
Happy painting, happy eBaying!