NINJA BLADE

There are 20 emblems. Some need to be collected in the levels, and some are given as reward when you complete many conditions through the game.

1. Oni-Slayer Blade : in mission 1, can be collected after you use the Dual Blade grapple for the first time, in a barrel.
2. Kurokawa Yakuza : unlocked with only 1 Sword used in a mission.
3. Number 1 : unlocked with a score of 14k (or so) in a mission.
4. Shuriken : in mission 2, you have to wall run to reach the Slime Chopper. Emblem can be found in some barrels as you reach the platform where it retreats from you, before using the Dual Blade grappling.
5. Dual Falcon Knives : in mission 3, after obtaining the spark ninjutsu and behind the metal bars where gunners are shooting you (as seen in the cutscene).
6. Evil : unlocked by finishing a mission with 0 Retries.
7. Centipede : in mission 4, in the room between the two wings. You must collect it the first time you cut through the room.
8. Stonerender Sword : In mission 5, just before you grapple over to the 2nd Blastmite fight, in a dark Corner beside the train wreck.
9. Oni : in mission 6, hidden behind a breakable wall, after passing by the helicopter, crossing the office-like room, 2 wallruns, and before the grapple.
10. Raven : in mission 7, on your way to the wind shuriken, before you jump down to the 4 Rocketlauncher guys next to the red barrels, cross the bridge and destroy a crate to get it.
11. Waterspirit : in mission 7, on your way to the spark shuriken, when you enter a dark room with a giant plant to burn, the emblem is behind a breakable wall right hand of the plant.
12. Magatama : in mission 8, hidden on the first platform you fight on, in a barrel.
13. Shinobi : Unlocks after doing a 100 hit combo.
14. Warrior's Creed : Unlocks after obtaining every Chi Piece.
15. Ninja Blade : in mission 9, at the first fork, take the left pass and check for a breakable wall after doing a pole jump. Emblem is behind it.
16. Endless Flowers : It unlocked for me after finishing the game in the normal difficulty, but i'm not sure of the condition required.
17. The Ninja : unlocked by finishing the game with 0 retry at overall count.
18. Invincible : unlocked by finishing the game in the hard difficulty.
19. Love: unlocked by completing all the levels using only one type of sword to attack (see that's all I need achievement).
20. Sword : it unlocked for me at the same time as the Ninja Blade outfit.

You might have to do the mission 9 again to unlock the sword emblem (or another emblem even if it's the last mission you did), and the achievement.