Go into the Blocks palette and open the Block (EC button).
Select the object using the "Edit Facet" tool.
Select a facet, then Ctrl+A to select all the facets.
Go to Properties and choose a different Category.
Leave "Material" at "None".
You should now be able to select the object and change its material in the normal manner. "Finish Edit Content" to the Block.
Designer, Deluxe, (Professional, Expert, Basic), Platinum
Windows 10 Pro (1903), 64-bit