There is little known about the Blackfyre Rebellions, which took place much before the events of Game of Thrones. It began when Daemon Blackfyre laid claim to the Iron Throne after his father, King Aegon IV Targaryen, legitimized all his bastards on his deathbed.
Game of Thrones: The Blackfyre Rebellions, Explained
The Blackfyre Rebellions were the longest ongoing civil dispute in Game of Thrones, causing the Seven Kingdoms about “five generations of strife”.