It is represented by a blue circle with a skull in the middle. The skull has a black-eyed skull on its forehead.
The Black Eye Skull can be found on "Crow's Nest" on the normal or higher difficulty. When you begin the level look up, and you will see a large pipeline hanging from the ceiling. If you look further down again, you will see a red fencing, which can be jumped on, from there you can already see the skull at the end of the pipeline.