If you are on *nix, look at the file using od -xc filename. When you say that a line is "blank" what do you mean? I suspect you mean that this position in the file is taken up by a newline ("\n" on Unix, "\r\n" on Windows). This is only a single character, and if you opened the file for read-write ('+<') then you would still have a problem of fitting 6 characters ("hello\n") where there is only room for one.

Text files on UNIX and Windows are just a single stream of bytes, they are not physically split into records. The "lines" which we see are only a convention and convienient way of viewing them.