98% of the time, the problem is the scale of the object that is displaying as black. Sometimes, it appears the object has proper scale but one of its parents is passing down a small scale. Some CAD files have a scale that is far from 1 since it was using different units than what Create uses. For example, Create uses cm and if a model made in mm units comes in the result would have a scale of 0.1.
Normally this scale is only set to the parent object upon importing.

If the parent has a smaller scale, than 1, the child objects will inherit the parents scale even if it shows they are set to 1 in Create. The child will only show the true scale once it's been moved to the root of the hierarchy.

For example, in Create scale being at 1 means its 100% scale of its immediate parent in the hierarchy. However, in the screenshot below, the “Assembly Parent” object was at 0.000039 for scale which means each child has a true scale of 0.000039 even if a child object like “Sub Part 01” shows its scale is set to 1. This is too small and will cause its material to show as black.

To Do
1. Ensure your CAD when imported is using cm units and all objects are at 100% scale.
2. Import the new CAD file with cm units to your Asset library.
3. Use the 'Replace Asset' function to replace your CAD in the project.