Who may enter a love relationship with Blackwall? You can enter a relationship with Blackwall only if you play as a woman, regardless of your race.
How to build positive relations with Blackwall? During all of the conversations that Blackwall takes part in (regardless of whether you talk eye to eye, or with more characters), pick the dialogue options that suggest sincere and just approach. Apart from that, try to be merciful towards others (e.g. when you need to decide the fate of a captured opponent). Also remember to pick all of the dialogue options marked with the heart.
More important steps:
- The first conversation with the Grey Warden takes place after you move to Skyhold. Blackwall notes that your courtships are not that innocent and he considers them out of place. Do not worry about that and search for opportunities to pick the dialogue options with the heart.
- Alongside improving your relations with Blackwall, also complete the successive parts of his first personal quest- Memories of the Grey. To complete this quest, you need to visit various locations and you gain access to some of them only after you have reached Skyhold.
- After you have completed "Memories of the Grey", you need to improve your relations with Blackwall, to the point at which he invites you on a journey to Storm Coast, during the Explanations side quest. Complete the quest.
- Some time after "Explanations" Blackwall disappears from Skyhold and you will not be able to pick him as the active party member. Do not worry about that, because this is connected with his last side quest- "Revelations". Go to Val Royeaux to complete the quest.
- While completing "revelations" you need to meet with Blackwall in the prison and return to Skyhold, to make a judgement over the Gray Warden. If you are still interested in the relationship with him, you need to do either of the following - allow him to set out to meet Gray Wardens, or let him decide his own fate. Finally, meet him at the stables in the Skyhold to enter a fully-fledged relationship with him.