Use the Windows API function SetBkMode(). Example:

procedure TForm .Button Click(Sender: TObject);
var
  OldBkMode : integer;
begin
  with Form .Canvas do begin
    Brush.Color := clRed;
    FillRect(Rect(0, 0,  00,  00));
    Brush.Color := clBlue;
    TextOut( 0, 20, 'Not Transparent!');
    OldBkMode := SetBkMode(Handle, TRANSPARENT);
    TextOut( 0, 50, 'Transparent!');
    SetBkMode(Handle, OldBkMode);
  end;
end;