Vantablack, absorbing 99.44% of all visible light.
 

I found some videos on YouTube. Some imitate Vantablack (so us mere mortals can have a version). Except for the one where a guy shines a ridiculously powerful flashlight on it, it looks fake:

https://www.youtube.com/watch?v=DI7tLclZyrE
https://www.youtube.com/watch?v=9v0_fID_jvA
https://www.youtube.com/watch?v=BkGHwrq2Eho
https://www.youtube.com/watch?v=AaFdCvnV8PM

Does this have any application for us?

Radiator. Objects that are great at absorbing energy are also great at radiating energy. Only problem is that radiators mostly conduct/convect heat away, not radiate it. Painting it black will make a negligible difference.

I've never thought this before, but radiators are misnamed. They should be convectors or something.

That took me on a tangent- Thanks Xist

https://www.youtube.com/watch?v=C4h-AS729JM