a little black book

There is absolutely no reason to assume that it refers to blackmail. If that assumption is made by you based on context, you need to provide the context if you expect a reasonable answer.

"A little black book" most commonly refers to a small booklet of blank pages sold for the purpose of writing down the names and addresses of friends of the purchaser. By extension, it has come to mean any list of persons which someone may make. For example, one might warn someone else that "i'm writing you down in my little black book," meaning that the person addressed has transgressed in some manner, and is being added to a list of persons whom the owner of the little black book does not like.

There is no earthly reason to assume it refers to blackmail.

according to Answers.com, the definition of "black book" is
i've always thought of a "little black book" as a place where one keeps the names & phone numbers of people one might wish to date...

Sometimes, metaphorically, a "little black book" would refer to a bachelor who has a lot of girlfriends. One might say, " I wish that I had John's little black book", meaning, I wish that I had so many girlfriends.(who are listed in that book)

> i've always thought of a "little black book" as a place
> where one keeps the names & phone numbers of
> people one might wish to date...

same here.

We aren't getting confused with BAD books here, are we? Getting in someone's "bad books" (plural) for example your boss, means being noted as unsatisfactory by that person. There is no need for an actual book.

--------------------------
Ayn Rand was a DICK.

That is a gross an unwarranted slur on Dicks everywhere . . .