Why i cant combine my Black Suit with Legendary Armor Pieces while i can use it with this ugly blue Vault Suit?

u can get a mod that will let u do that =D

MAN! I WISH!!!!

'I am on console'

If you do a few quests for the Railroad then you can unlock the ability to add damage and energy resistance to black suits (and other select clothing)...at armorer 3 I'm able to put a 90/90 damage/energy resistance on suits.

Its not about the Damage Resis its more abt the Legendary Stats like less VATS Points and so.

Because armour would not clip properly with suit. Only dress which is modeled with armour in mind can have armour layers added. That leaves all dress with jackets, coats and similar out.