Thanks for the information, Dutchbird.
I thought you meant the items provided, and I could not find any black-grounds.
Do the following:
Click on the item in the Asset Browser,
RMC (right mouse click) on the item> Show Info
Go to the Info tab, RMC on the Preview> "Update Preview from File".
Choose the image you like to use, typically a Dome light rendered square-sized image (jpg)
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To my knowledge, there is no other way, as any extra item in the Asset Scene used for the background would populate the Object Manager after loading it.
Enjoy