declare
    vblob    blob;
    voffset  integer := 1;
    vbufchar varchar2(3200);
    vbufraw  raw(32000);
    vbufsize binary_integer := 3200;
  begin
    dbms_lob.createtemporary(vblob, true);
    for j in (select * from temp_table_dropme)
    loop
        for i in 1 .. ceil(dbms_lob.getlength(j.pfdata) / vbufsize) loop
          dbms_lob.read(j.pfdata, vbufsize, voffset, vbufchar);
          vbufraw := utl_encode.base64_decode(utl_raw.cast_to_raw(vbufchar));
          dbms_lob.writeappend(vblob, utl_raw.length(vbufraw), vbufraw);
          voffset := voffset + vbufsize;
        end loop;
    end loop;

    update temp_table_dropme set base64new = vblob;
  end;

declare
  v_blob blob;
  o1 integer;
  o2 integer;
  c integer;
  w integer;
begin
    for i in (select * from temp_table_dropme)
    loop
      o1 := 1;
      o2 := 1;
      c := 0;
      w := 0;
      dbms_lob.createtemporary(v_blob, true);
      dbms_lob.converttoblob(v_blob, i.pfdata, length(i.pfdata), o1, o2, 0, c, w);
    end loop;
  
  update temp_table_dropme set base64new = v_blob;
end clob2blob;

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...
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  declare
    vclob    clob;
    vblob    blob;
    voffset  integer := 1;
    vbufchar varchar2(32000);
    vbufraw  raw(32000);
    vbufsize binary_integer := 32000;
  begin
    for j in (select * from temp_table_dropme)
    loop
        -- убираем CR и LF
        dbms_lob.createtemporary(vclob, true);
        while voffset < dbms_lob.getlength(j.pfdata) loop
            vbufchar := replace(replace(dbms_lob.substr(j.pfdata, vbufsize, voffset), chr(13), null), chr(10), null);
            dbms_lob.writeappend(vclob, length(vbufchar), vbufchar);
            voffset := voffset + vbufsize;
        end loop;
    
        -- декодируем clob
        dbms_lob.createtemporary(vblob, true);
        voffset := 1;
        while voffset < dbms_lob.getlength(vclob) loop
          vbufraw := utl_encode.base64_decode(utl_raw.cast_to_raw(dbms_lob.substr(vclob, vbufsize, voffset)));
          dbms_lob.writeappend(vblob, utl_raw.length(vbufraw), vbufraw);
          voffset := voffset + vbufsize;
        end loop;
    end loop;

    -- vblob
  end;
/