I also noted that if you set TargetType to the block, it only triggers for local blocks. I ended up switching up to using TargetType = typeof(ContentData) and then check in editor descriptor what type of content is being created. That works for shared blocks. Example to hide LinkUrl property on contact block for alloy site:

    [EditorDescriptorRegistration(TargetType = typeof(ContentData))]
    public class MoveFormsPropertyEditorDescriptor : EditorDescriptor
    {
        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
        {
            var contentDataMetadata = (ContentDataMetadata)metadata;
            if ((contentDataMetadata.Model as ContactBlock != null))
            {
                foreach (var metadataProperty in metadata.Properties)
                {
                    var mp = metadataProperty;
                    if (mp.PropertyName == nameof(ContactBlock.LinkUrl))
                    {
                        mp.ShowForEdit = false;
                    }
                }
            }

            
        }
    }