Caption: Purcell's hohlraum is a toy version of an experimental hohlraum (see also Harvard Natural Sciences Lecture Demonstrations: Purcell's hohlraum).

You too can make your own Purcell's hohlraum from any completely closed carton of some kind.

Seal the carton thoroughly, put a black sheet of paper on one side (NOT essential, but a nice touch), and punch a small hole in the black paper.

Virtually NO light reflects out of the hole in hohlraum and the radiated EMR from hole has approximately the blackbody spectrum for room temperature.

The blackbody spectrum is invisible to the human eye, and so all you see is blackness---blacker than the black paper:

There's something about this that's so black, it's like how much more black could this be? And the answer is none. None more black.
---Nigel Tufnel in This is Spinal Tap (1984 film).

It's probably NOT as perfect a radiator of a blackbody spectrum: i.e., a blackbody radiator.

However, it's probably NOT so bad in that regard too.

So any relatively small opening to enclosed dark region is probably a reasonably good blackbody reflecter, but usually NOT a good blackbody radiator.

So fairly good blackbody reflecters are common in everyday life.

Interfaces between materials that turn up in everyday life are probably NOT very good blackbodies since they usually are measurable reflecters.

The blackest materials known circa 2017 are superblack and vantablack.

One of the most perfect blackbody spectra in nature is the cosmic microwave background (CMB).

In controlled experimentation, a very perfect blackbody spectrum can be obtained from an experimental hohlraum.

Credit/Permission: © David Jeffery, 2012 / Own work.
Image link: Itself.
Local file: local link: purcell_hohlraum.html.
File: Blackbody file: purcell_hohlraum.html.