The Dojo section of Fruit Ninja is the place to view your unlockables and extra blades. Below is a list of the items that you can unlock in the Sensei's Swag section of the Dojo:
Original Blade: This is your starting blade.
Shiny Red Blade: You unlock this upon first checking Sensei's Swag.
Disco Blade: Slice 50 bananas.
Mr Sparkle Blade: Slice three pineapples in a row while playing Classic Mode. TIP: This is very difficult to get and you are relying on the game's fruit placement to be kind to you. If two pineapples come up together, slice them both and then hope that the next fruit up is another pineapple. Note: in the Lite version, you must finish a round on exactly 42 points (42 being the meaning of life) to get this particular blade.
Old Glory Blade: Complete a game having earned exactly 50 points.
Butterfly Knife Blade: Get a combo involving a strawberry 40 times.
Flame Blade: Get a combo after the bell rings while playing Zen Mode.
Ice Blade: Get 20 Freeze Bananas while playing Arcade Mode.
Shadow Blade: These achievements are different between iOS and Anroid, so look at the correct achievement for your device.
iOS: Achieve a final score of 175 while playing UPSIDE DOWN in Zen mode. TIP: To make it so that your device is able to turn upside down, you need to toggle 'orientation lock' in your iOS device's settings. If you go into Sensei's Swag and look for this achievement, you will notice the upside down bit is in red. Turn your device upside down and it should go green, showing that you can now earn the achievement.
Android: Achieve a final score of exactly 234 in Arcade Mode (while playing the right way up). TIP: If you are struggling with this one, try aiming for a finish of 204, as you normally get a 30 points bonus at the end of the round. Keep an eye on what your normal bonuses are and then aim for a final score of 234 minus those bonuses.
Pixel Love Blade: Get 50 combos while playing Classic Mode.
Piano Blade: Slice 100 critical hits.
Party Time Blade: Slice every strawberry but nothing else while playing Arcade Mode.
Bamboo Shoot Blade: Play a full Zen Mode game every day for five days.
Firecracker Blade: To get this secretive blade you must earn the same score as the year of the Battle of the Red Cliffs. This was 208 AD. So you need to get 208 points in Classic mode.
Note that all of the above are for the FULL version of Fruit Ninja unless stated otherwise. The Lite version requires different criteria to get the unlockables, and also only features a selection of those listed above.
Original Wood Background: This is unlocked automatically.
Fruit Ninja Background: Earn 125 points without dropping a fruit in Classic Mode.
I Heart Sensei Background: Read 3 different strawberry fruit facts. Note: To get these fruit facts, play in Zen mode and slice a majority of strawberries and you should receive a fun strawberry fact.
Great Wave Background: Slice 250 Watermelons.
Yin Yang Background: Slice 75 Passion fruits.