I know
Even if DG doesn't like it lol it IS canon now that you can go for 30 years before having to heed your Calling. The Warden and Alistair have only been tainted for 10 years, and even if Blackwall is a veteran, could've only been 20 years for him.
How it is canon? The fact that Alistair said it work like this (even without considering his actual inexperience in Warden stuffs at the time) doesn't mean much. He said he never assumed lyrium and that templars don't need it, which was already proven wrong/retconnetted. The same could go for the Calling, if the writers decided it works like Gaider said (at best 30 years, and worse during the Blight).
Regardless, we don't know how much time Blackwall was a Warden. If he's 50 and was recruited when he was 20, he could still be near the Calling.
I don't really want him (or the Warden and Alistair) to go through the Calling during the game, but we have no clue on how close he could be to his Calling regardless of the two theories on the taint/Calling.