I've just realised that CC is "carbon-copy" and BCC is "blind-carbon-copy". Basically I'm wondering, where did these terms come from?
Back when typewriters were in common use and photocopiers were rare, one kind of paper you used to be able to buy actually came as two or more sheets stuck together at the top with carbon paper between each sheet. This way you automatically had multiple copies of whatever you put on the paper. This was commonly used to save typing work for office memos, but was especially common for forms. If they were filled out by hand, they'd put instructions on the top of the first page to "press firmly when writing" to ensure even the bottom copy was legible. I think such forms are still around in some places, but they now use specially backed paper instead of separate sheets of carbon paper. Forms also typically had different words at the bottom of the carbon copy pages saying who got that copy.
Now the "blind carbon copy" I always thought was an email innovation. However, Unreason in the comments below found a reference to it from a secretarial handbook from 1979. Wikipedia explains it was made to emulate an old office typist trick of adding addressees to one (or more) of the carbon-copied sheets after the copies were made, so that the others don't see that addressee. According to the handbook, secretaries were supposed to make note of these extra recipients in a file, but the other recipients would be "blind" to the fact that these extra people received copies.
I always thought it was "carbon copy" (and upvoted that answer) but I just read a letter to New Scientist http://www.newscientist.com/article/mg21028171.200-cc-all-readers.html saying otherwise. The letter-writer points out that Latin used double letters for plurals, a habit that came into academic English -- pp for pages and LLB for the degree Bachelor of Laws, for example. (As a student I used ff for following in my own notes, eg "p 21ff has quick summary".) Apparently cc would thus mean copies, not carbon copy. The letter claims that cc predates carbon paper or even typewriters, and was used in medieval times. Intriguing and at least slightly convincing. Note it's never c.c. or CC - always cc. To me that adds a little support.
Here is something I found:
The letters CC stand for Carbon Copy or Courtesy Copy. The acronym was originally established when carbon paper was used to produce one or more copies simultaneously during the creation of paper documents on old typewriters. This technique declined in the advent of the digital age when documents were created and distributed electronically. The well known acronym cc, or c.c., for Carbon Copy was changed to mean a Courtesy Copy
When Typewritter along with carbon paper was actually used letters used to have at the bottom of the page, a list cc: Names, Initials, Department, Or Notice Borad ID for people/Departments who were getting copies. The name of the addressee(s) was not changed. At the time of actual distribution, a tick mark was placed by Pen against a specific name in CC to ensure that no one got the list twice by error and no one was missed.
In certain cases, it was required that a senior or unrelated person needed to be informed. His name would be written out/typed on the additional copy. It was written BCC so that the receiver also knew that he is got a copy without others knowing about it.
Carbon copy is correct. The OED defines c.c. as "carbon copy or copies (followed by a list of others to whom correspondence is to be copied)." The earliest citation listed is this: "1936 L. I. Hutchinson Standard Handbk. Secretaries 287 The carbon copy notation, ‘c.c.’, should be the last notation." The definiton of bcc (no periods supplied) is "blind carbon copy" and the earliest citation is: "1974 W. H. Bonner Better Business Writing 455 You would omit the carbon copy notation from the original, but would typewrite the letters bcc (meaning blind carbon copy) on the carbon copies only."
It is, however, true that both terms were in use earlier. A search of Google books turns up a reference to the term in a 1912 U.S. government report stating that "by almost universal practice of business concerns the carbon copy has supplanted the press copy as a record of the outgoing correspondence." Press copies were made by wetting the original and pressing it onto another sheet, and this was the first widespread technology employed to make duplicate copies of correspondence. Reference to carbons replacing the press method is clear evidence about where the term came from.
The earliest blind carbon copy turns up in Google Books is 1948 in The Secretary's book: a complete reference manual.