Create a firework like I did in the screenshot. Following the setup I used, light it off under the Crafting table and a graphical glitch will appear.

If you make the fuse time longer, place one under the table and cover the opening in front of it (but not under it) the blue flash will appear black instead.

Sometimes it will seep through the block so it might take multiple tries to get it.

First screenshot is the firework I used for all the tests I did.

Appears to be black if the space the firework is lit off in is dark or unlit.