To zenithal with a black base or not, that is the question. (A waffle)
When you paint miniatures they're subject to levels of Abstraction and Stylisation.
What does this mean?
Abstraction is how 'non real' you paint the thing. The more something looks 1:1 real, the less abstraction. But even things that seem on the surface like an attempt to look real contain a lot of abstraction.
The artificial lighting you add, using metal paints to suggest metal, adding edge highlights and black lines to make the mini read well, even layering paint to make highlights, all are a non real, abstractions.
You can see this in the classic art world with a lot of old oil works striving for realism, getting looser and more abstract with impressionism and stepping really far away with cubism.
So what is stylisation?
Stylisation is a kind of a catch all for a mostly deliberate/consistent form of abstraction. If you paint Boarderlands looking minis, or cell-shade or Blanchitsu, you have chosen to translate the real though a specific lens of abstraction to get a look you are after. A style.
What's this got to do with black based Zenithal?
Without going heavy into the reasons, shadows are not really made up of black. They usually have colour. A mix of the materials and the ambient (a soup of reflected light from the environment and nearby objects)
Using black to as the base, tends to make colours murky and desaturated in an unnatural way.
This does not make it wrong. But being informed is important. If in your miniature painting you seek to make the most realistic looking models possible. This this is a blind ally. Turn back.
If however you like (or are ok with for speed reasons) the look. Then this is a level of abstraction and stylisation you are comfortable with, or even actively seek.
So, To zenithal with a black base or not, that is the question...
The answer is: depends
No if you are seeking absolute realism.
Yes if its a style you are activity attempting to achieve.