You can send a wm_SysCommand message with the WParam parameter set to SC_MonitorPower and the LParam parameter set to 0 to turn the monitor off, and - to turn the monitor back on. The following example turns the monitor off for a 0 second interval. Example:

type
  TForm  = class(TForm)
    Button : TButton;
    Timer : TTimer;
    procedure FormCreate(Sender: TObject);
    procedure Timer Timer(Sender: TObject);
    procedure Button Click(Sender: TObject);
  private
    { Private declarations }
  public
    MonitorOff : bool;
    { Public declarations }
  end;
var
  Form : TForm ;
implementation
{$R *.DFM}
procedure TForm .FormCreate(Sender: TObject);
begin
  Timer .Enabled := false;
  Timer .Interval :=  0000;
  MonitorOff := false;
end;
procedure TForm .Timer Timer(Sender: TObject);
begin
  if MonitorOff then begin
    MonitorOff := false;
    SendMessage(Application.Handle,
                wm_SysCommand,
                SC_MonitorPower,
                - );
    Timer .Enabled := false;
  end;
end;
procedure TForm .Button Click(Sender: TObject);
begin
  MonitorOff := true;
  Timer .Enabled := true;
  SendMessage(Application.Handle,
              wm_SysCommand,
              SC_MonitorPower,
              0);
end;