Jisho

真っ暗 is a な adjective and 中 is a noun, so the な is used to place them together grammatically.

Are you familiar with な adjectives?

Thank you. Boy, do I feel silly.
I got hung up on 暗い as an い adjective and couldn't get beyond that....

.

Is there any way to know which ～い adjectives remain ～い adjectives after 真っ？ With colors, for example, it seems that 真っ黒い and 真っ白い are correct, but 真っ赤、真っ青、 真っ暗 are な adjectives. The category of ～な adjectives seems to have changed over the years; when I first started learning Japanese in 1960 we were taught that 大きな and 小さな were the only correct form for those (and some others) which are now commonly 大きい, 小さい, etc.

@DR

"when I first started learning Japanese in 1960 we were taught that 大きな and 小さな were the only correct form for those (and some others) which are now commonly 大きい, 小さい, etc."

Huh, that's very strange to me.

Miyamoto Yuriko, a writer who died in 1951, wrote a short piece called 大きい足袋https://www.aozora.gr.jp/cards/000311/files/16005_30002.html

Here's a short piece by Dazai Osamu, who died in 1948, which uses 大きい 4 times.
https://www.aozora.gr.jp/cards/000035/files/52526_45475.html

I have a hunch that whatever you're describing is more about the Japanese teaching resources of the time than anything concrete about the language.

真っ暗 is a keiyo-doshi （形容動詞）, and when it is work with a noun, it takes な as shown in your example.
真っ暗＋な＋中（中＝noun) I mean keiyo-doshi + noun.

However, 神様達は真っ暗い中で also sounds OK. I see this one is adjective (形容詞）+ noun.

If the past tense in "were the only correct form" is supposed to refer back to like the 14th century, then that might be more or less correct.

⑴室町時代以後、「大きい」が生じたのは、古代からある対義語「小さし」の存在による。すなわち、「大きなり」（形容動詞）↔「小さし」（形容詞）の組よりも、「大きい」（形容詞）↔「小さい」（形容詞）の方が、より対称性がはっきりするからである。「高し」（形容詞）↔「低ひきなり」（形容動詞）から「高い」（形容詞）↔「低い」（形容詞）という組が形成されたのも、同様に考えられる。
⑵「大きい」が成立すると、もとの形容動詞「大きなり」は、連体形「大きなる」が「大きな」という連体詞の形で、連用形「大きに」が副詞の形で（関西の感謝の言葉「おおきに」もこれからきている）残存した。連体詞「大きな」が成立するとともに、それまでなかった対義の連体詞「小さな」も成立した。
(from 小学館 精選版 日本国語大辞典 © Shogakukan 2006)

But damn, 1960 that is almost 65 years ago. Appreciate the life-long hustle, dai-dai-dai-senpai DR様