"How do the opposite colors, yellow and blue, mix into black?" This is how nature does it.

The pigment spectrum, the light spectrum in computers, the light spectrum in prisms and the light spectrum of transparent crystal colors made by color elements are all tightly related.

LEAD MAKES TWO OPPOSITE COLOR CRYSTALS ONLY, BLUE AND YELLOW

This color wheel is based on crystals, pigments are crushed crystals or chemicals relating to the colored elements. Crystals show the full range of each color. Yellow darkens to brown before it becomes black with one element, as in the titanium Rutile crystal. Titanium is a centering element that goes from Yellow to Orange, than Red to Brown and Brown to Blue in the Rutile crystal.

WAVELLITE, Al3[PO4]2[OH,F]3.5H2O, H3.5, SG-2.36, orthorhombic crystal system, acicular, like the needles of Rutile suspended in clear crystal, only these needle crystals form a dense rocklike mass.
ALUMINUM, yellow, brown, blue, centering standard, transparent. Aluminum really does it big this time, each needle starts at the center radiating outward, concentrical ringed bands form around the center, each band ending as a cleavage line, twenty or so bands per complete needle crystal. The band itself can change from clear transparent to opaque white while at the center, the second band range is transparent yellow crystal, than brown, then blue. From brown to blue, this crystal can change into an aggregate and lose it's crystal properties.
THIS MINERAL PHOSPHATE SHOWS THE CENTERING OF YELLOW TO BROWN AND BROWN TO BLUE, LIKE MY REAL COLOR WHEEL FOR PIGMENTS.
http://www.realcolorwheel.com/crystal.htm#63,%20WAVELLITE,
There are many crystal color examples on that link.

Brown lightens to yellow, yellow darkens to brown, like a banana or the Wavellite crystal. The computer colors get dark by subtracting light, in that respect it's different than pigments.
Opaque pigments choose one of the many colors of yellow available, yellow, Naples yellow, yellow ocher, brown ocher.

1705, COLOR,
Bister was a transparent yellow to brown, dual-tone color. It was made from charred beechwood and was mainly a water-color pigment.

Some transparent modern chemical pigments go from a light brown to yellow, Original and synthic Indian Yellow Br/s and Org/s do it.

What I'm saying is, opaque brown and yellow are two parts of the same color as far as my color wheel or crystals are concerned. Mix dark yellow (brown) with ultramarine blue and you get a perfect dark neutral.

Don