This all-black varient of the US flag has been popping up since the start of the pandemic and was seen at J6 among other places. It can be seen flying from the porches of some of the worst Trump and DeSantis supporters. It has been especially popular in Florida for some reason

It stands for “no quarter” and specifically for “no quarter given” meaning an intention to slaughter all defeated combatants in the second US civil war they expect. This is distinct from “no quarter asked” in which case an intent to refuse to surrender and fight to the death is stated without changing the status of enemies captured if those making this statement are victorious. The combination of the two is “no quarter asked or given” no surrender and no prisoners taken.

It is generally considered a war crime to refuse to accept the surrender of defeated enemy troops (as the all black US flag threatens) while it is perfectly legal in war for combatants to refuse to surrender themselves. Of course, once either side gains a reputation for either executing or torturing captured enemies, the whole conflict can quickly escalate to no quarter asked or given on either side, as happened between US and Japanese troops during WWII’s “Island-hopping” hell.

During the (1st) Civil War the Confederates sometimes used a no-quarter black flag, and it was after the civil war that US statues concerning conduct of the US military outlawed declaration that no quarter would be given. Troops are still permitted to refuse to surrender, they just must accept the surrender of defeated enemies and not kill them.

https://www.miamitimesonline.com/opinion/the-no-quarter-flag-and-its-threat-to-america/article_0e7221c0-46eb-11ec-8195-e7b68bed1e86.html