The "Fifth Edition Character Sheet" app actually refers to the Bladesinger Arcane Tradition.
The Bladesinger is a new Wizard Tradition detailed in the Sword Coast Adventurer's Guide by Wizards of the Coast. It can be found, specifically in page 141 of the said book.
It should be noted that this app labels weird names when presenting certain class options. Allow me to list a few examples:
- "The Undying" Warlock Patron is listed as "Undead"
- "Sun Soul" monks are listed as "Light"
- "Purple Dragon Knight" Fighters are listed as just "Knights"
Oddly, Swashbuckler and Mastermind rogues, Battlerager Barbarians, Arcana Clerics and Oath of the Crown Paladins are named correctly. To deal with these inconsistencies with the app, you have to have a firm grip of 5e class options, as even options like Sorcerer's Metamagic options, Warlock Eldritch Invocations, and a Battlemaster Fighter's maneuvers are all paraphrased and not listed verbatim.
You can take solace in the fact that the app has no homebrew or Unearthed Arcana added in it, so everything there is official material which can be easily found.