I’ve been seeing social media posts of people doing this. Not sure what the background behind it is.
Edit: Here’s an example of what I mean: http://www.walmart.com/ip/All-Black-American-Flag-3x5-Ft-US-Flag-2-Pack-UV-Protection-Flags-Heavy-Duty-Embroidered-Stars-Sewn-Stripes-Brass-Grommets-Outdoor-New/828247324
What does the black American flag mean?
At this point, there doesn't seem to be one set definition for the meaning of a black American flag. In general, black flags are used by enemy forces to signify that no quarter will be given. What that means is that, during war, enemy combatants are going to be killed rather than taken prisoner. Right now, that meaning doesn't neatly line up with what the black American flag might mean.
https://www.distractify.com/p/what-does-a-black-american-flag-mean