Frost / Blades of the Fallen Prince Hidden AppearanceThe Dark Runeblades hidden appearance for the Blades of the Fallen Prince for Frost Death Knight is unlocked by using the item Runes of the Darkening.
The location or drop place for the Runes of the Darkening is currently not known, so check back often for updates, or go search for yourself.
The Dark Runeblades color tints:Default: Green.
Marine: Complete 100 dungeons with the Dark Runeblades equipped.
Purple: Complete 200 World Quests with the Dark Runeblades equipped.
Red: Kill 1 000 enemy players with the Dark Runeblades equipped.
Unholy / Apocolypse Hidden AppearanceThe Unholy artifact weapon hidden appearance is called the Bone Reaper, and is unlocked by using an item called The Bonereaper's Hook. The location for this item is still unknown.
The Bonereaper's Hook color tints:Default: Dark Blue.
Blue: Complete 100 dungeons with The Bonereaper's Hook equipped.
Red: Complete 200 World Quests with The Bonereaper's Hook equipped.
Icy: Kill 1 000 enemy players with The Bonereaper's Hook equipped.
Blood / Maw of the Damned Hidden AppearanceThe hidden appearance for Maw of the Damned is called Touch of Undeath, and is unlocked by using the item called Twisting Anima of Souls. In order to get your hands on this item, you need to go to Suramar for the Quest Quest / Withered Army Scenario. Use the quest Withered Army Training to start the scenario.
If it's not available for you, keep doing the Suramar questline until you reach this point. You also need to have at least 400 Ancient Mana ready before you can go. Here's a short guide on how to farm Ancient Mana extremely fast. Note that 400 Ancient Mana is the minimum, more mana will get you more soldiers and a higher chance to complete the scenario with a higher score.
The Twisting Anima of Souls drops from the final chest at the scenario, but some people think that the higher your score is, the better the drop rate for the Weapon Appearance is, so you better try as hard as possible to get a good score. If you don't get it, farm some more Ancient Mana and return later.
Twisting Anima of Souls color tints:The default color tint for the Twisting Anima of Souls is blue, but you can also unlock the following color tints:
Red: Kill 1000 players with Blue appearance equipped
Green: Complete 100 dungeons with Blue appearance equipped
Purple: Complete 200 world quests with Blue appearance equipped.