Bolt's arm has no more spotted black than Reed's arm. But you know what? It doesn't matter. Black Bolt's costume is not blue, regardless of what even Jack Kirby may have done in a given image in a given page of a given comic 40 years ago. Highlights of Black Panther's costume weren't typically rendered as gray (although his costume was black/gray in Steranko's Captain America run) because as it was explained in the letters pages at the time, gray "muddied up in print" and was unusable.

This all comes down to limitations of the product. A person might as well ask why Star Trek characters Anne Mullhall, Miranda Jones and Katherine Pulaski all look like they were played by the same person. Television has limited casting, comics have (or had) limited palettes.

People either get it or they won't...

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Exactly right!

Comics are a language of shorthand, and it behooves the readers to play
along with that shorthand, not to question it. Unfortunately, there is a
strata of fandom that, pretty much from the beginning, has demanded
logic where logic simply could not exist (largely due to the
limitations of the form). Sadly, as the audience has shrunk, the numbers
of these particular fans have remained largely constant, and so their
percentage in the populace (and therefore their "volume") has increased.

••

There we run into a different question. Was it dark brown because it was
meant to be dark brown, or was it dark brown because that was the color
they could be sure would photograph as black?

In the early days of color TV, a lot of tricks were needed to get the
images to look as they were supposed to. On BONAZA, there was a crew
of painters whose job was to spray down all the trees in the foreground
of exterior shots with yellow! This was not to give the impression the
Ponderosa had trees with yellow foliage, but to be sure they looked green
on the home screen! Similarly, George Reeves' Superman costume in the
black and white episodes was actually shades of brown, to be sure
distinct shades of gray would appear on screen. (The values of the red
and blue in Superman's costume are so close they ten to appear as the
same shade of gray in black and white -- the red of his chest emblem
almost disappears against the blue of his shirt.)

••

Again, here I go with the color it appeared on screen.