Because it's the first time I've seen that advice/critique, why is using solid black on a character outside of lineart a bad thing?
To be frank, it truly depends on what you're aiming for.
For example, in the powerpuff girls, black is commonly used in characters.
Difference between them and hazbin is that they use black as a legitimate addition to the colour palette while hazbin uses it to cheat. This as well as using multiple other shades of black leads to a ineffective colour palette.
So heres the base we'll be using, hands, horns, and boots. Common victims of hazbins black out issue.
[srry if the quality is shite]
Often these are completely blacked out, with the exception of white detailing. This can cause confusion on what an object is, so odd white lineart is used to differentiate between things, like fingers.
Another common issue spindlehorse has is the lack of cohesion in what black theyre using, often using up to three different types of black. This is seen in characters like husk, lucifer, stolas and blitz.
Of course multiple shades of black CAN work in a character design, but these are HAND ANIMATED SHOWS.
Keeping track of all these shades of black would be HELL.
This is easily fixed by using one shade of black throughout the character. It links the blacks together and relates it to the character so that each part doesnt look so out of place.
Surprisingly, they CAN do this, as seen with ALASTOR of all characters
For similar reasons, you should never use solid white in a design, though due to it being so light it'll never have the same issue of lacking important detail.
But more importantly, if you use off-whites and non-solid blacks, you can actually add more dimension and detail to these areas, for extra ✨special✨ scenes!
I hope this explained it somewhat! of course theres other more effective ways of explaining my points, but hey, hope it helps!