TDSC = packed record
    dsc_dtype    : byte;
    dsc_scale    : shortint;
    dsc_length   : word;
    dsc_sub_type : smallint;
    dsc_flags    : word;
    dsc_address  : pointer;
  end;
  PDSC = ^TDSC;

  TISC_BlobGetSegment = function( BlobHandle : pointer; Buffer : pChar; BufferSize : word; var ResultLength: word ) : integer; cdecl;
  TISC_BlobPutSegment = procedure( BlobHandle : pointer; Buffer : pChar; BufferLength : integer ); cdecl;

  TBlob = packed record
    GetSegment       : TISC_BlobGetSegment;
    BlobHandle       : Pinteger;
    SegmentCount     : longint;
    MaxSegmentLength : longint;
    TotalSize        : longint;
    PutSegment       : TISC_BlobPutSegment;
  end;
  PBlob = ^TBlob;

{ Тестовая UDF, возвращающая размер BLOB-а }

{
DECLARE EXTERNAL FUNCTION TEST_BLOB_SIZE
    BLOB BY DESCRIPTOR
RETURNS INTEGER BY VALUE
ENTRY_POINT 'UdfBlobByDescriptor' MODULE_NAME 'MyTest.dll';
}

function UdfBlobByDescriptor( var Param : TDSC ) : integer; cdecl; export;
var
  Blob : PBlob;
begin
   Blob := PBlob( dsc_address );
   Result := Blob^.TotalSize;
end;

 Blob := PBlob( Param.dsc_address );