What is the ruling on hoisting a black banner on the minaret on Friday morning, then replacing it with a white one at the time of the Adhan? Is this part of the Sunnah or an innovation (Bid'ah)?

Placing a black banner on the minaret on Friday morning and lowering it before the Friday Adhan to replace it with a white banner has no basis in the Sunnah. It is considered a reprehensible innovation (bid'ah qabihah) that must be warned against and avoided.