If you make a piece of armor an upgrade it to legendary, can you improve the same armor again when you have all the pieces of equipment that have the max Fortify Smithing Enchantments?

Or do you have to make a complete new set of armor and wear all the equipment with the max Fortify Smithing Enchantments?

Also, its it possible to enchant a piece of equipment that will cover the armor in a black smoke like the Ebony Armor you receive from Boethiah?

Yep, you can re-improve armor and weapons if your smithing skill gets higher.

Nope, the Ebony Mail effect is unique to that.

IMO,smithing is useless.

Cheers

IMO, blanket statements without arguments (that are also so wrong that it makes my toes curl) are useless.