What? Communists, at least Marxists, ARE anarchists in a way. The main difference between them and what we usually call Anarchists is that communists believe we need to create a transitional system of socialist government that will slowly "wither away" into a communistic society - a moneyless, stateless society. i.e. a form of anarchy. Anarchists believe that no state can ever be trusted and we should overthrow the current system directly to a anarchic system. Of course there are many types of anarchists but the ones most people think of are "Anarcho-Communists". There are other types like Anarcho-Capitalists which are like even more extreme libertarians, but that's even more niche. The biggest, most typical group that use red/black flags are Anarcho-Communists.