The quest becomes available for female Inquisitors who have flirted with Blackwall on several occasions.
If the Inquisitor chooses the flirt option during the cutscene with Blackwall on the ramparts (following their initial arrival at Skyhold), he will reject her advances, claiming that their relationship will not work. Some time after, a conversation triggers in which Blackwall claims he wishes to explain his reasons for pushing her away, but not at Skyhold.
Travel to the Storm Coast and follow the quest marker to Blackwall's chosen location. The closest camp for quick travel is Small Grove Camp. When you are near the area, Blackwall will say that this was the location in which he and another Warden were ambushed. When the compass begins to pulse, use the search button to locate the hidden object. A cutscene occurs in which Blackwall picks up the object and explains that it is the Warden-Constable's Badge.
Regardless of the options the player chooses on the dialogue wheels, the quest will end with Blackwall saying that he needs time to think and that they should talk more at Skyhold.
I tried to save him, but he died.
The Warden-Constable's badge.
This was my life before I met you. Crumbling ruins. Endless battles. Death.