真っ暗 (makkura) means “completely dark”

More examples

山道は日が沈むと真っ暗になります。san dō wa hi ga shizu mu to ma kkura ni na ri ma su

The mountain path becomes completely dark when the sun sets.

It presents darkness as total and leaves no visible detail in the surrounding space.

真っ黒makkuro: Describes something with an entirely black visible appearance.

あたりに光がなく、何も見分けられないほど暗い状態。a ta ri ni hikari ga na ku nan mo mi wa ke ra re na i ho do kura i jō tai

真っ暗 (makkura) is built from 真 (ma, "true" or "complete") and 暗い (kurai, "dark"), using 真 as an intensifier for a total lack of light.

In everyday Japanese, 真っ暗 (makkura, "completely dark") can modify a place directly, as in 真っ暗な部屋 (makkura na heya), rather than describing only the time of day.