There’s a theory out there that this is an acronym, standing for Book Layout And Design but I think that’s just an imaginative post hoc rationalization.  Quite apart from the improbability of coming up with exactly these words to describe the item in question, there was a perfectly decent and appropriate word in existence long before the practice of making blads afflicted the publishing industry.

A blad, described by the Oxford English Dictionary as chiefly Scottish, is 1) a fragment, a portion, or 2) a portfolio, or 3) a pad of paper or blotting paper.  In 1933 Eric Partridge records the word’s usage in something like our sense: “Blad . . . is applied to a sheaf of specimen pages or other illustrative matter liked by the bookseller, especially the bookseller resident abroad”.  The place of residence of this bookseller does seem to confuse things a little: I suppose what he may be implying is that it would be cheaper to send a few pages overseas than the entire book.  By 1960 the word’s meaning was exactly ours: “a sample of a book, made up for the publisher’s traveller to show to the trade.  It usually consists of the first 32 pages, including prelims, bound up in the same cloth as the finished book”.  It was immediately after this that blads enjoyed their heyday. When international co-editions became the way to afford four-color printing, blads became the required way of selling them.