So dark you can't see your hand in front of your face.
Tips
usage
伸手不见五指 (shēnshǒu bú jiàn wǔ zhǐ, 'stretch out your hand and can't see five fingers') is a set phrase meaning 'pitch black.' Also, 向人伸手 means 'to ask someone for money/help' — it carries a slightly negative tone, implying dependency.
