Effect of black coating on the inside of a hobby telescope's tube?
Telescopes have the inner part of the tube blackened to minimize scattered light reaching the eyepiece. Such scattered light reduces contrast, and hence reduces the ability to see very faint objects, or low contrast detail.  A complementary approach for refractor telescopes is to use baffles in the tube. The baffles form a physical barrier to scattered light.  However baffles are costly to make accurately and add to the weight of the telescope.
The more that scattered light is reduced, the better the performance.
Alternatives to super absorbing paint are to attach to the tube, either matt black velvet, or sand paper sprayed with matt black paint.