"Blackwall You can recruit Blackwall after The Lone Warden quest. You get that quest after you go to Val Royeaux and talk to the Templars. Go to the Hinterlands and do the quest and tell him you want him to join you. Blackwall is a warrior and starts off with a sword and shield."
I'm not sure you lost the chance 'cause I recruited him as soon as it was available.
"The Lone Warden is automatically triggered as The Threat Remains is resolved. You are to look for a man named Blackwall in the Hinterlands. Travel there, specifically to the Upper Lake camp. The search area for Blackwall is just southwest of the camp.
Blackwall can be found on the west side of the lake; you'll need to go around since your character apparently doesn't know how to swim. Once you're there, you'll find him addressing some peasants. Talk to him, and you'll trigger an easy fight. Afterward, you can choose to bring Blackwall into your party. Doing so will also unlock a new mission at the war table. Either way, the quest will resolve."
Try this, if you don't find him could be a bug Idk