The idiom "black book" refers to a list of people who are considered untrustworthy, dishonest or disreputable. It is usually used to refer to a group of individuals who have been known to cause trouble for someone.

To reveal a secret or a hidden truth, often unwittingly or unintentionally.

The idiom "learning curve" refers to the gradient of a concept or skill as it is learned, where initially it is difficult to grasp but becomes easier over time. It describes the process of acquiring knowledge or skills and how it gradually improves with practice and exposure.

The idiom "come out" can have a literal or figurative meaning depending on the context.

In its literal sense, "come out" means to emerge from a hiding place or a closed space into an open area. For example:

• The cat came out of the box.

The idiom "yellow streak" refers to a person who is extremely cowardly or timid, lacking courage or bravery in situations that require them.