This is the 9334th most frequent Japanese word.
真っ
"真っ" (まっ) means "pure," "entire," or "completely," often used as an intensifier in compound words.
Here, '真っ' emphasizes the intensity or absoluteness of the following word, '青' (blue). It conveys 'completely' or 'purely'.
真っ青な空が広がっている。
A completely blue sky is spreading.
In this sentence, '真っ' describes the darkness ('暗') as total or complete, intensifying the adjective.
真っ暗な部屋に入った。
I entered a completely dark room.
Here, '真っ' enhances '直ぐ' (straight), indicating perfectly straight or direct without deviation.