Blow by blow, much like play by play, is an idiom used to describe a step by step process or a detailed description. This phrase also sometimes appears with hyphens between each word. It is a quick way to ask for more or fewer details when another person is giving an account of an event that previously transpired.
A commonly used saying is, “I don’t need a blow by blow description.” This indicates that the person addressing the situation is giving a very detailed description and the person using this expression would prefer a quicker, simpler explanation. On the other hand, someone may wish to know every detail. “Slow down. Give me a blow by blow ; don’t skip anything.”
The phrase "blow by blow" is an adjective meaning that something is explained in minute detail. Etymologists believe the term has been in use since 1933. "Play by play" tends to refer to the description of a game or other similar sporting event. Blow by blow may have originated as a term referencing the sport of boxing, as in “giving a blow by blow of the fight.” "Blow" is a word used to describe a hard strike, such as those dealt by a boxer. Such a description of a fight would be a more literal interpretation although the phrase is commonly used in less than a literal sense like many other idioms.
A blow by blow description does not merely relate details but should also do so in the chronological order in which they occurred. A police officer might ask for such a description in reference to an automobile accident or when interviewing a person who witnessed the commission of a crime. A journalist might also ask for a blow by blow account from his or her sources, or give such a description of an event when giving his or her report.
Idioms also can be, and often are, used as titles. This particular one is the title of a Jeff Beck album as well as a song by Fleetwood Mac. There is also a book entitled, Blow by Blow: The Story of Isabella Blow.