BLFS is just a term for all that you can do with your system after you finish LFS (hence the name beyond LFS). There's an BLFS Book (similar to the LFS book) which discusses common things people do, such as setting up more networking, XFree86, email, firewalls and so forth, but BLFS isn't limited to what the book contains. The BLFS book just has a very few things compared to what people do with their systems.
You may only pick a few things from the blfs books and lfs-hints. Then you'd go on installing something not covered by the book. That's still in the BLFS realm even though the book ends somewhere.