lots of variables into black bark and how it will taste.
I generally prefer a dark mahogany color for my bark, I don't find black bark appetizing.
Some rubs will blacken and taste just fine, others taste better when blackened, and some will taste like shit when blackened.
The more sugar a rub has, the worse it tastes when black.
If your rub has sugars, try to keep it in the mahonany zone