[rgstuamigo]

Aqui te pongo el codigo de un componente (adaptado por mi) descendiente de TMemo que se pone trasparente cuando no tiene el foco..

Código Delphi [-]
unit AlphaMemo;
interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls;

const
  TWM_RGSInvalidate=WM_USER+1;

type
  TAlphaMemo=class(TMemo)
  private
    { Private declarations }
    procedure RgsInvalidate(var Message:TMessage); message
              TWM_RGSInvalidate;
    procedure CNCTLCOLOREDIT(var Message:TWMCTLCOLOREDIT); message
              CN_CTLCOLOREDIT;
    procedure WMEraseBkgnd(var Message: TWMEraseBkgnd); message WM_ERASEBKGND;
    procedure WMMove(var Message: TMessage); message WM_MOVE;

  protected
    { Protected declarations }
    FTransparent: boolean;
    procedure CreateWnd; override;
    procedure CreateParams(var Params: TCreateParams); override;
    procedure DoExit; override;
    procedure DoEnter; override;
  public
    { Public declarations }
    constructor Create(AOwner: TComponent); override;
    procedure Invalidate; override;

  published
    { Published declarations }
  end;

procedure Register;

implementation

constructor TAlphaMemo.Create(AOwner:TComponent);
begin
inherited create(AOwner);
   ftransparent:=true;
end;

procedure TAlphaMemo.CreateWnd;
begin
inherited CreateWnd;
   if fTransparent then
      begin
       SetWindowLong(Parent.Handle, GWL_STYLE,
       GetWindowLong(Parent.Handle, GWL_STYLE) and not WS_CLIPCHILDREN);
       end;
end;

procedure TAlphaMemo.RgsInvalidate(var Message:TMessage);
var r:TRect;
begin
  if (Parent<>nil) and FTransparent then
    begin
      r:=ClientRect;
      r.TopLeft:=Parent.ScreenToClient(ClientToScreen(r.TopLeft));
      r.BottomRight:=Parent.ScreenToClient(ClientToScreen(r.BottomRight));
      RedrawWindow(Handle,nil,0,RDW_FRAME+RDW_INVALIDATE);
    end;
end;

procedure TAlphaMemo.CNCTLCOLOREDIT(var Message:TWMCTLCOLOREDIT);
begin
  if FTransparent then
   with Message do
    begin
      SetBkMode(ChildDC,Windows.TRANSPARENT);
      Result:=GetStockObject(HOLLOW_BRUSH)
    end
   else inherited;
end;



procedure TAlphaMemo.WMEraseBkgnd(var Message:TWMERASEBKGND);
begin
  if FTransparent and not (csDesigning in ComponentState) then
      PostMessage(Handle,TWM_RGSInvalidate,0,0)
  else inherited;
end;

procedure TAlphaMemo.WMMove(var message:TMessage);
begin
inherited;
if FTransparent then SendMessage(Handle,TWM_RGSInvalidate,0,0)
  else Invalidate;
end;

procedure TAlphaMemo.CreateParams(var Params:TCreateParams);
begin
inherited CreateParams(Params);
if (CsDesigning in ComponentState) then exit;
    with Params do
      begin
   ExStyle:=ExStyle or WS_EX_TRANSPARENT;
      end;
end;

procedure TAlphaMemo.DoExit;
begin
inherited;
   FTransparent:=true;
   SetCursor(0);
   RecreateWnd;
end;

procedure TAlphaMemo.DoEnter;
var ExStyle,StdStyle:longint;
begin
inherited;
Ftransparent:=False;
StdStyle:= Windows.GetWindowLong(handle, GWL_EXSTYLE);
ExStyle:= StdStyle and not WS_EX_TRANSPARENT;
Windows.SetWindowLong(Handle, GWL_EXSTYLE, exStyle);
Invalidate;
end;

procedure TAlphaMemo.Invalidate;
begin
if FTransparent then SendMessage(Handle,TWM_RGSInvalidate,0,0)
  else inherited;
end;

procedure Register;
begin
  RegisterComponents('MyComponents', [TAlphaMemo]);
end;

end.

Solo copia y pega el codigo en una nueva unidad ,lo guardas y luego lo instalas.
Saludos...