Hidden Duality: How to get level 2's blue boomerang?

If I'm not mistaken, head right out of that room with the boss door and key, go south a few rooms and head back to the main entry room of the dungeon, drop down behind the right torch in the second room with the mice to the lower floor. Bomb the left wall on the lower floor and head west. Head north into the chest containing the second key needed to access the boomerang. The boomerange itself is to the left of the boss column of rooms if that makes any sense.