In the main tower of Surface Central Area, you can exchange any 3 swords for Black Sword. The attack power of Black Sword will equal half of the sum of 3 swords' attack. Stamina requirement is always 10, and the sword is always dark elemental. Thus, you should use strong but impractical swords (due to stamina cost) to obtain Black Sword.
Strongest possible Black Sword (73 attack; possible in Chapter 5): Invincible Sword + Buster Sword + Blaze Blade
However, obtaining Black Sword in Chapter 4 will make the rest of the game much easier. Invincible Sword + Golden Sword + Buster Sword gives 70 attack Black Sword. If you don't have Buster Sword, then just use another Golden Sword for 62 attack Black Sword.
If you Sacrifice Rick's Byroad (40), The Buster Sword (48), and the Invincible Sword (60), you get a 74 attack Black sword, Major drawback of losing Rick's sword though.
Also All swords can be upgraded with the Goddess Roulette where the assistant can add 5 attack to Rance's current weapon, can also be combined with Strength Spray as well for an 80+ attack