How to get HFS+ filesystem blocksize
You could try "stat" with the "%k" format option.  According to the man page this is the "Optimal file system I/O operation block size".  Command line is:

stat -f "%k" FILE


Where "FILE" is any regular file on your system (e.g. "~/.profile"). It should say "4096" or something like that.  I cannot 100% guarantee this is what you're looking for, but it seems close.