http://pubs.opengroup.org/onlinepubs/009695399/basedefs/sys/stat.h.html says "A file system-specific preferred I/O block size for this object. In some file system types, this may vary from file to file.", which says essentially the same as the Linux stat(2) manpage from which I copied the extra words.

The same page claims that st_blocks may use other units than 512 byte blocks, but that seems to be quite rare. GNU coreutils sources claim HP-UX and AIX PS/2 have non-512 blocks. Perhaps it would be better to indicate how to find out the block size? (Since st_blksize is not it, but that's an easy assumption to make.)