Where can I find the story of the black dragons in the dragon's punch bowl?
"Pathfinder Chronicles: Dragons of the Sandpoint Hinterlands". It is located at the end of Pathfinder Comic, Volume 1 #8. It features black dragons Black Fang and Scarhorn, as well as a bronze dragon Tiruvinn.
They is also one paragraph about them in the first book of Jade Regent AP, "The Brinewall Legacy".