(as) black as (one's) hat old-fashioned simile
Completely black; totally dark
This place looks so creepy, it's as black as my hat!
I can't see anything in this tunnel, it's as black as my hat.
Completely dark, without any light or color
Completely black, without any light or other color
Completely dark
Meaning absolutely certain or without a doubt
I am as sure as God made little green apples that Peter's the one for you, you two make a beautiful couple.