This part of the Handbook is Work-In-Progress.
libblockfs is a support library in Managarm responsible for handling file I/O on block devices like hard drives and USB. It is a general interface to the underlying file systems that are available in Managarm. Userspace programs do not call libblockfs directly, but request functionality through the posix subsystem.
The code for libblockfs can be found at https://github.com/managarm/managarm/tree/master/drivers/libblockfs.