Former relationshps would partly depend on when he was recurited, I'd guess.
If I remember correctly in DA:O one of those recruited had a wife, pregant with a child.
He might have a wife with a child - who he had to leave or who left him due his wardening business keeping him too busy and away from her. Or she might have cheated on him while he was away. Or found someone new, thinking he was dead...
Or he might have found a companion among the wardens, who died in the recent blight. Or heard hear 'calling' early.
All posibilities :3
I admit I have - not once - considered Blackwall as a romance option to this point. To me he had a sort of 'Duncan' vibe. Awesome character, but more of a mentor personality who is not interested in romantic relationships anymore. Also, I feel Blackwall might be bittersweet with being a warden and not among the youngest anymore... :\
But I'm honestly interested in getting known to him as a companion. And who knows, once I do, I might go all heart eyed for him.
That said. Have we ever heard him talk? In Bioware games I'm apparently a sucker for voices. Most of the romances I played or wanted to play were decided by voices first ^^°