Eight fifteen-letter answers, spanning the entire puzzle. Just 18 black squares.
Tough puzzle to get into, but once it got going, it wasn't that bad. Our esteemed colleague Rex Parker hated this puzzle because of a single cross, but fortunately for us, Frances knew that MULETAS were (28A: Matadors' red capes), so MINAS just had to be right for (28D: ____ Gerais (Brazilian state)).
That cross aside, there was some nice cluing of crosswordsy fill like (18A: They often pass through needles) for SERA, (22A: Rounded-up numbers?) for HERDS, and RIOTS for (19A: People might leave them in tears). I also enjoyed HOPIN as a (23A: Driver's invitation). Never heard of a NIT being a (38A: Small grouse), but MENISCI (31D: Crescent-shaped bodies) came to me from my high school chemistry classes. I think we were supposed to read the bottom of the meniscus curve when measuring fluids. Is that right? Any chemists out there?
The one answer I strongly disliked was BIDIN (39D: Act to retain one's property at auction). Shouldn't that be BIDoN? You "bid at" an auction and you "bid on" an item in an auction. You don't "bid in" anything.