When you duplicate the blinds design you will not be able to select this as a design type within the PowerGallery admin area. All that you are doing is taking an existing design and duplicating it so that you can make customizations without altering the original.
To make use of this new design you will need to follow along in the document and rename the folder and the references to the design in the include code that you would like to make use of this newly customized design.
Here is a direct link to the How To for reference:
custom_slideshows.php