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.