A compound that absorbs all visible light
Substances which absorb almost all the light falling on them appear black. Therefore you are looking for the blackest known compound.

The record is currently held by Vantablack[1], a substance composed of vertically aligned carbon nanotubes, which absorbs up to 99.965% of visible light incident upon it. As you can see it is really black.



Previously, the record was held by Superblack[2,3], a nickel-phosphorus alloy with an absorbance of up to 99.65%.

For comparison, regular black paint generally absorbs around 97.5% of visible light.



Sources

[1] https://en.wikipedia.org/wiki/Vantablack

[2] https://www.newscientist.com/article/dn3356-mini-craters-key-to-blackest-ever-black

[3] http://pubs.rsc.org/en/Content/ArticleLanding/2002/JM/b204483h#!divAbstract