It should work - it does for us.
I had the same problem as Katharina, but after fidling a bit with the EditorDescriptorBehavior that was set to OverrideDefault for our CustomCategoryEditorDescriptor and setting it to just Default, as below, Joels code worked in Epi 10 :)
TargetType = typeof(CategoryList),
EditorDescriptorBehavior = EditorDescriptorBehavior.Default
I have implement this article http://joelabrahamsson.com/hiding-episervers-standard-category-property/ in my another projects which has episerver cms < 9. They worked perfectly. But for my lastest project using episerver cms 10, that code not working.
Anyone has ide how to hide a category property in block using cms 10?