procedure TForm1.atDiagram1InsertDControl(Sender: TObject; ADControl: TDiagramControl);
begin
  if UpperCase(ADControl.DControlID) = 'MYBLOCK' then
  begin
    With ADControl As TDiagramBlock do
    begin
      Shape := bsNoShape;
      Metafile.LoadFromFile('C:\MyMetafile.wmf');
    end;
  end else
  if UpperCase(ADControl.DControlID) = 'MYOTHERBLOCK' then
  begin
    With ADControl As TDiagramBlock do
    begin
    Color := clYellow;
    end;
  end;
end;

initialization

RegisterDControl(TDiagramBlock, 'MyBlock', 'This is a caption for my block', 'My category');
RegisterDControl(TDiagramBlock, 'MyOtherBlock', 'This is a caption for my other block', 'My category');

atDiagram1.GraphicLib := dglGdiPlus;