Every performer knows the definition of blue material. It is performance material, usually sexually explicit, “dirty” or curse words. To work “blue” means that you use that kind of material. It has also used as the term “blue law” which puts limits on performances.
But, why do we call it “blue” or “blue material”?
In the late 1800s, industries such as steel, traction, gas, and oil were being consolidated into monopolies. Vaudeville was no different. One of those who controlled the theaters was B. F. Keith.
The Keith-Albee circuit owned the most important theaters and the most theaters. Only a handful of performers had the fame or power not to follow the Keith-Albee rules and if you didn’t follow the rules, you didn’t work.
As Keith expanded his theater chain and as the bills were expanded to continuous, all-day Vaudeville, he needed to clean up the normally crass performances for an audience of more than just men. Mrs. Keith took on the task.
“At her behest, all manner of blasphemy and lewdness was expunged.”
First, “notice to performers” signs listed the don’ts and don’ts for onstage performances.
Later, managers for the organization would send “suggestions” back to the performers in little blue envelopes. The performer must then change the offending material or be barred from the circuit.
That is why performers today use “blue” to describe any kind of offensive or barred material.
The quote and background comes from, " No Applause--Just Throw Money, The Book That Made Vaudeville Famous" by Trav S.D. This is a terrific history of Vaudeville. You should click over to Amazon.com and buy it. Now.