Like the standard scoped .44 magnum, the Blackhawk is a powerful, accurate sidearm. It deals more damage than a DKS-501 sniper rifle, and holds a single bullet more. It has no recoil when looking down the scope.
Upon completion of Agatha's Song, you can bring Agatha a sheet music book. She will offer the Blackhawk as a reward. To get the weapon in full condition when you receive it, be polite to Agatha when both accepting the quest and receiving the pistol after completing the quest. You must use the reward speech option to get the Blackhawk. Picking "Yes, anything that you can afford" will get you the Blackhawk without a karma change; picking "No, no, a reward won't be necessary" will give you positive karma but no Blackhawk.