Sounds like you ran into an agent (#1) who is not familiar with the Black Out dates on some partner airlines. NW has no BO dates, but merely zeroes out the availability buckets for W and A classes if they don't want to give away any free seats on a flight. It appears that Korean leaves the seats in the bucket for flights during Black Out periods, but they will just sit there because they can't be used.

The scenario would probably have gone like this, if you had actually tried to confirm the reservation with Agent #1:

"OK, I have the seats reserved for you. Could you hold on a moment? I can not get my computer to price this correctly for me."
"Hello, Rate Desk, what am I doing wrong? I can't get this reservation to price correctly #@!##$!"

Rate Desk Person: "That's because you forgot about the Black Out dates! Sorry, but Rules are Rules, so no mileage tickets can be sold on that flight."

"Hello, sir/madam, I am very sorry to say that you (and countless others) can not redeem your miles for something that you really want. But could I interest you in 3 free tickets to Minot???? Or perhaps you would like to use "double miles" and fly on Northwest to Manila????

You can call back as many times as you like, but the final answer will always end up being "NO". And BTW, welcome to Flyer Talk.