真っ暗で

because it’s pitch-dark

→ 真っ暗だ
phrase Rare #45,520
because it’s pitch-dark 真っ暗 means “pitch-dark”; で connects this state to what follows, often giving a reason.
JA: 部屋が真っ暗で、何も見えない。
EN: The room is pitch-dark, so I can’t see anything.

真っ暗でよく見えないわ

It's pitch-dark; I can barely see.

— 崖の上のポニョ (2008)

真っ暗で分からないの

I can't tell because it's pitch-dark.

— レオン (1994)

真っ暗で何も見えやしない

It's pitch-black; I can't see a thing.

— 崖の上のポニョ (2008)

ねえ真っ暗で怖いわ

Hey, it’s pitch-black and scary.

— レオン (1994)