When a blank slide is inserted in a program it does not display the background selected in the blank slide Display Properties as expected.
The key bit of information is that the blank slides are affected by the properties of the Blackout settings in the various SSP Display Properties. You will find that the default setting for the Blackout is set to "Use Color" and it is set to black.
Check the Global Display Properties settings of the Blackout properties. If you set the Blackout properties to "Use Background" it will use whatever background you choose - motion or still. This will affect all blank slides unless specifically overridden with a program or individual item Display Property setting.
If you do not want to override the Blackout settings globally, you can set the Blackout properties to "Use Background" and select the desired background in the Display Properties for a particular blank slide. This will only affect the blank slide with the properties adjusted in this manner.