So first up Goddess throws an unfortunante person into a new bad situation and he thanks her? Theres no way the goddess predicted this would happen this way cause she'd have had him be the uncles kid from the word go.
Also if black magic is just more then 1 attribute there is no way this society wouldn't know this. If it's just a special thing for him that'll be different but if all Black can use anything it fully breaks the world building cause people would figure this out real quick and Black would be the desired colour over all of them.