Modify horizontalrule plugin (Add Inline Style)

I would like to modify the horizontalrule Plugin and add some Inline Style into hr.

I tried to add black background (in plugin.js) to <hr /> but its not working:

canUndo: false, // The undo snapshot will be handled by 'insertElement'.
        exec: function( editor ) {
            var hr = editor.document.createElement( 'hr' );
            hr.style.background = "#000";
            editor.insertElement( hr );
        },

Can anyone help me out with this?