When you reach World 3-6 Outlook Point, you'll be able to buy the Secret Blue Door Sticker from the monopolizing Toad for a mere 80 coins. Better stock up - there's a Blue Door to place in almost every level. Most of the time, the doors hidden in the levels house Things, but occasionally, you'll have some rare Stickers sneak in as well or an awesome +5 HP upgrade. Below are a list of all of the locations where you can Paperize the Blue Sticker Doors into the landscape.
Move through World 1-1 until you reach the faint outline of a door under a sign with a Hammer Sticker on it. Paperize the Blue Door Sticker on it and you'll be rewarded with a Thumbtack Thing!
As you're getting close to the finish line, you'll notice that the huge platform that holds the Sticker Comet has the door outline you're looking for. It's hard to miss - Paperize it when you find it. Inside, you'll find a Flashy Ice Flower Sticker, a Flashy Fire Flower Sticker, and a Flashy POW Block, all of which are great additions to the Sticker Museum.
Go through the level and remove the Faucet as you did the first time through the level. When the water spout is gone and you can jump in the hole underground, you'll spot the door outline right behind you. Inside is a Bed Thing. Also, if you jump onto the bed, a lullaby is played and you regain your HP.
When you pop out of the red warp pipe (just before the finish line), you'll notice a door outline to the north. Paperize a Blue Door Sticker onto it to collect the Matches Thing inside.
Take the shortcut warp pipe that you discovered the first time you passed through the level (near the beginning area). When you're underground, you'll see the faint outline of the door near the middle of the path. You'll be rewarded with the Bellows Thing for your good hard work.
In the part of the fortress where the two giant fans blow you off the bridge, drop down to the bottom and you'll notice a Door Outline to the left of three blocks. A Squirt Gun Thing rests inside.
When you reach the pyramid where the Sticker Comet rests, work your way to the top level of the back side (you'll have to use the sides of the pyramid to help you climb. A Shy Guy is blocking the door outline, so get him out of the way and you'll find the Car Battery Thing inside.
Head around to the back side of the Sphinx where the HP-Up used to be, and you'll be able to find the door outline in plain view. Head inside to collect the Paper Fan Thing.