I'm re re watching the Season 9 opener and noticed, as Sombra's cloak is flowing out behind him while he's laughing at the mane 6 about the destruction of the Tree of Harmony, he's a Blank Flank.

What if THAT was what made him turn to darkness?

What if he was being teased for it by other ponies so he tried to find a short cut to get one, or sought out power to get back at those who made fun of him, and instead of getting a Cutie Mark, he got power over darkness, shadow, and fears?

One cat.
One tree
Six ponies
No batters
Only JUDGEMENT

Judge of harmony coming soon.