Correct me if I am wrong, but doesn't the taint generally take 30 years for it to fully spread? If so, what is to say Blackwall has been a Grey Warden since he was 20? He could have easily been in his thirties or even forties before he was initiated, which would leave plenty years before his Calling. Maybe, maybe not.
So I would argue to stop making assumptions, since there isn't any real evidence to bolster it as far as I am aware. Even if Blackwall is in that age where the Calling is near, if BW actually manages to execute its presence properly through his romance and as a character in general, then I won't have any problems with it. What I found lacking with Thane romance was not his death but how poorly it was handled.
Gaider later clarified that 30 years is the best a Warden's lifespan can be, but not all live for that long. The taint might take some Wardens before. Plus, we don't know yet how exactly the Blight works in increasing the taint process. Does it works for every Warden, or only those who are near the Blight?
Blackwall was a Warden for at least 10 years, since he was during the fifth Blight. He probably was for more then 10, since he was the second-in-command at that time. I guess he's been a Warden for at least 15-20 years. Though i don't really have an opinion of his arc in DAI, if it'll be around being near the Calling or not.
On another topic, do you think we might get some interesting banter between Leliana and Blackwall? Maybe the latter is curious about the Warden.