Hi everyone.:)
I have discovered that while you are in Denerim, do not accept any Blackstone Irregulars quests before doing Back Alley Justice for the Chantry.
I found out that if you do, one of the three alleys will not display on the City Map, which means you cannot travel there to complete the Chantry quest.
The solution is to do Back Alley Justice first so that all three alleys will be displayed.:)
You might also want to make note of the Mages Collective quest (where you mark four homes). I have not tested that one, but it also has part of it in the alleys.