You will need to Group the Backings for any of the templates you make on your end for your clients in your Admin Builder mode.
Your client will have access to the backs once you group the front with your chosen sides.
The backside will indeed come out blank for the end-user if you do not group them.
The first step is you need to choose the template to be the "back" when you are created it. The system will then know it is a backside.
The next step would be to group the chosen backs with the front. You can get to the grouping window by click on a Frontside template and choosing Edit Groups.
The next step would be to create a "Layout Group." Once this is done you can select the "Backs" that you want to be associated with the Selected Front Side Template.
At this point, you can select which back sides show in which order, however, this is not necessary.
You are now finished, and you can now return to layouts.