Here's the company I used for my DD's testing.
All it did was confirm that she is African, Asian and Caucasian. But in the case of your child... if his father is black it will definitely tell you the truth no matter what he looks like since his mom is CC.
It's a good test IMO if you don't know much to start with....but some people don't care for it because it tends to open a can of worms and gives you more questions to ask. My test results did open a can of worms and I had to go back and look at my grandparents with new eyes.
AncestryByDNA
The darkening thing relates to cuticles and ears. Kids will typically match those areas eventually. My DS changed to match his... my DD's didn't have to change because she was already the color of her ears and cuticles.