<p>
paulewog,
</p>
<p>
Not sure what a blank custom slide does that clicking the close-with-theme doesn't.
</p>
<p>
However if you need to, there is away to add a blank custom slide. In the title and body fields, hold down the left ALT key whilst typing 0160 on the numeric keypad (right hand of the keyboard).
This will enter a special space character which will fool openlp.org into allowing you to add the slide
</p>