Well that’s a different question. Not to put words in your mouth, but the same tone Nolan uses and a serious tone aren’t the same thing so that’s why I got tripped up.
There are black directors who use a serious tone but it’s not the same tone as Nolan because his subject matter is wildly different. Most, not all, but most black directors dabble in real life stories or historical fiction. Rarely do they do something like inception or the prestige which has a serious tone but is heightened reality. Do the right thing is a serious movie but it’s not the same tone as the dark knight, which is also a serious movie.
Subject matter plays big in tonality and they don’t play in the same sandbox as Nolan.