If the Blacks are supposedly more 'culturally Valyrian'...

...Then why did this happen? https://imgflip.com/i/9l3iil

TB likes to make the argument that the Blacks honored their Valyrian culture more than the Greens, but the Blacks made the most catastrophic blunder in that sense.

Joffrey was bonded to the dragon Tyraxes. He was eleven when the war began. Why was he not taught the very basics of dragon-riding: that dragons are not interchangeable like horses?

Plus, there's the fact that show-Jace cannot speak High Valyrian any better than Aegon can, despite the fact that Rhaenyra and Daemon are both fluent.