You must either be "in" the directory of the file, or use the full path to the file when trying to cat it. Also cat does not open a file in libre office, unless there are some special options I am not aware of, all it will do is dump the contents of the file out onto the console. You can also use cat to add lines of text into a file but that is not what you are trying to do either.

Most likely you can just type libreoffice /home/$USER/path/to/blog.odt and that will open the file in libreoffice. Note that $USER needs to be replaced with your name if it is not set as an environment variable. And /path/to/blog.odt needs to be the path from your home folder, not your desktop, to where ever blog.odt is.

For example if it was in a folder called "blog_stuff" on your Desktop, and your username was bob, the command would look like libreoffice /home/bob/Desktop/blog_stuff/blog.odt

Let tab completion become your friend in this, that is hitting tab twice while typing a path, it will either auto complete or show you the list of possibilities that match what you have typed so far.