My pleasure, Noble!
I think the issue is monitor resolution. It occurs on my 13″ Macbook regardless of the level (World, Continent, or Kingdom). It happens on my Win10 desktop at Continent or Kingdom level because of the number of hexes and numbers.
You can also try changing the line-width of the grids under the Show Grid gear. If I change it to “.5” the mini-map isn’t all black, but it looks like I’m looking at it through a screen door. I usually just keep the grid and numbers unchecked unless I need them for short-term reference on the main map. As soon as I’m done, I uncheck them again.