Yeah, Tamiya USA shows it sold out:
https://www.tamiyausa.com/search/?q=nato+black
I agree with Tojo, supply chains. Still disrupted as a result of the lockdowns.
If you're not wedded to Tamiya exclusively, it's a charcoal grey, so you might find an equivalent in some other brand's catalog. I like to use Tamiya's NATO Black for a "scale" black-it looks like black in scale distance, eg, looking at the model on a display table, which would equal 40 or 50 feet in 1/1 scale. But I can go to some other brands line Andrea or Vallejo (Model Color), or the craft store brands like Folk Art, Apple Barrel, Americana, etc, in a pinch.