Historically, the black flag is strongly associated with anarchism, anti-state politics, revolt, and rejection of national authority.

Had he colored it in the union jack, then I would've said it was nationalism, and the person is blinded by nationalism.

But. This is Banksy, black-and-white Banksy, so there may be no symbolism behind the black flag, but it's just very interesting. I can't accept that he would not have considered the color of the flag.