Blizzard is making Balance of Power artifact appearance rewards account-wide in Patch 10.0.7.

After unlocking the base appearance, the criteria for other color tints can be completed on any character in the forthcoming update.

(Source)

The Balance of Power questline artifact appearance rewards are now account-wide. After unlocking the base appearance, the criteria for additional color variants (such as the Glory of the Legion Hero and Unleashed Monstrosities achievements) may be completed on any character to earn the appearances.