Pure henna will only give a orangey-red color.
Indigo will give a dark blue color. Sometimes indigo is labeled as black henna even though it is not henna. Also, there are some nasty chemicals like PPD which is also called black henna. That stuff isn't henna either.
I color my hair and cover my grays with a mix of half henna and half indigo. This gives me a chocolatey burgundy brown color on my normally light brown hair. The indigo tones down the red and gives a more brown shade. The color intensifies and deepens with each application. The henna permanently bonds to the hair and doesn't just coat it so the hairs are a bit thicker. This helps to loosen my curls just slightly. Those with superfine hair might not like this effect though.
I have pics but not on this computer. I'll try to post them later.