The back is flat. Here is a side photo of the knob and the post. After clicking the link, click the right arrow beside the four photos under the main photo. Then you can click on the photo with side view and it will open up to a larger size. They are black with a hint of copper/bronze wash. However if you prefer they be completely black we can customize them for you.
https://www.patemeadows.com/Cirque-KnobFinial--Small--2-14-Diameter_p_354.html#.XdcmDtVKivE