[jpindar]

If you want to see something very black, look at the side of a stack of razor blades or x-acto knife blades. The individual blades are shiny, but the geometry of the stack makes it absorb almost all the light hitting it.

[ksynwa]

Found a small blog post about it since I was wondering what it looks like: https://erossel.wordpress.com/2015/08/24/beamdump/

[kazinator]

Another deep black is a small opening (the only one) into a large, hollow object.

https://en.wikipedia.org/wiki/Hohlraum

[MrEldritch]

This is, in fact, the same principle behind how Vantablack works!