Origin of Black Saints Clothes

Question

I wonder if there is any explanation in the anime or manga about the origin of the Black Phoenix Army and Black Saints clothes, since they are not constellations (as far as I know).

Answer 1

The origin of the Black Saints (暗黒聖闘士) was revealed in the data book Saint Seiya Cosmo Special published in 1988 (and re-edited in 2001), but it's canonicity is debatable. While it was aproved by Kurumada himself at some point, it wasn't directly written by him, and was published during the middle of the Poseidon arc weekly release, so there's plenty of later retconning that has some contradictions to what's shown on this data book. I don't think there's a full translation of said databook, but there's quite a few summaries around the internet, the one I'll be taking most info from is this one, while the introduction is only in Spanish, the actual content of the summaries are mostly both in Spanish and English, though there are some pieces that are exclusively on Spanish.

Around the time the Gigantomachia took place, the Continent of Mu (where the Saints Cloths were made) sank to the depths of the Pacific Ocean, with it, the knowledge of the crafting techniques along with the lives of the alchemists that created the Cloths were lost. But circa 1500 AD, an island was discovered by alchemists rebel to the Sanctuary that was apparently what remained of the Continent of Mu and was called Death Queen Island, there they started working on creating new Cloths, but since the knowledge was lost by the ages they could only craft imitations of existing ones, the most copied one being the Phoenix Cloth, the only real Sacred Cloth that still remained on that island and the strongest Bronze Cloth ever made. While their power could not compare to the originals, they were still powerful enough to be a worrying issue to the Sanctuary, especially since the warriors that claimed them were not Saints, but the warriors who were denied Saint status for they only gained a superficial understanding of the Cosmo and rebelled against the Sanctuary. As such, the island was sealed by the Sanctuary, and a keeper sent to guard the island and supress the Black Saints. By the time of the original Saint Seiya series, this keeper was Guilty, Ikki's master and Esmeralda's foster father. When Ikki killed him and broke his mask, the seal was broken and the Black Saints released