Is it possible for something to not reflect any visible light (for humans)? Eg can something only reflect infrareds or ultraviolets or gamma rays etc.?
It's an engineering problem, more or less, to develop a material that reflects no visible light. The closest we've come so far to a material that reflects no visible light at all is, afaik, Vanta-black, which absorbs 99.965% of visible light. Because it reflects almost no visible light it looks extremely black.
As far as I know there is no material that fully reflects all the frequencies of waves found in the electromagnetic spectrum, as energy is indeed minimally absorbed in all materials. On the other hand, a black hole which is almost perfect black body (Max’s theory) absorbs every single type of electromagnetic wave as light cannot displace under such intense forces. The blackbody absorbs any radiation ( here light) falling on it and appears black under reflection when illuminated from outside. When this blackbody is heated it radiates electromagnetic energy due to the thermal agitation of Electrons on it's surface.