create table blob_test( blob_content blob );

-- генерируем 1 МБ случайных данных
declare
   v_clob        clob;
   v_blob        blob;
   v_dest_offset integer := 1;
   v_src_offset  integer := 1;
   v_warn        integer;
   v_ctx         integer := dbms_lob.default_lang_ctx;
begin
   for idx in 1..1000
   loop
     v_clob := v_clob || dbms_random.string('x', 1000);
   end loop;
   dbms_lob.createtemporary( v_blob, false );
   dbms_lob.converttoblob(v_blob,
                          v_clob,
                          dbms_lob.lobmaxsize,
                          v_dest_offset,
                          v_src_offset,
                          dbms_lob.default_csid,
                          v_ctx,
                          v_warn);
   insert into blob_test (blob_content)
   values (v_blob);
   
   commit;
end;
/

set timing on

-- вариант 1
declare

  l_clob clob;
  l_blob blob;

  FUNCTION BLOB_TO_BASE64_NOAPEX (p_blob_in BLOB) 
  RETURN CLOB 
  IS
   v_clob clob;
   v_result clob;
   v_offset integer;
   v_chunk_size binary_integer := (48 / 4) * 3;
   v_buffer_varchar varchar2(48);
   v_buffer_raw raw(48);
   v_LENGHT NUMBER(9);
  BEGIN
   if p_blob_in is null then
    return null;
   end if;
   dbms_lob.createtemporary(v_clob, true);
   v_offset := 1;
   v_LENGHT := dbms_lob.getlength(p_blob_in);
   for i in 1 .. ceil(v_LENGHT / v_chunk_size) loop
    dbms_lob.read(p_blob_in, v_chunk_size, v_offset, v_buffer_raw);
    v_buffer_raw := utl_encode.base64_encode(v_buffer_raw);
    v_buffer_varchar := utl_raw.cast_to_varchar2(v_buffer_raw);
    dbms_lob.writeappend(v_clob, length(v_buffer_varchar), v_buffer_varchar);
    v_offset := v_offset + v_chunk_size;
   end loop;

   v_result := v_clob;
   dbms_lob.freetemporary(v_clob);
   return v_result;
  END BLOB_TO_BASE64_NOAPEX;
  
begin
  select blob_content into l_blob
  from blob_test;
  
  l_clob := BLOB_TO_BASE64_NOAPEX(l_blob);
  dbms_output.put_line( dbms_lob.getlength(l_clob) );
end;
/

-- вариант 2
declare

  l_clob clob;
  l_blob blob;

  procedure BLOB_TO_BASE64_NOAPEX (p_blob_in BLOB, io_clob in out nocopy clob) 
  IS
   --v_clob clob;
   --v_result clob;
   v_offset integer;
   v_chunk_size binary_integer := (48 / 4) * 3;
   v_buffer_varchar varchar2(48);
   v_buffer_raw raw(48);
   v_LENGHT NUMBER(9);
  BEGIN
   if p_blob_in is null then
    return ;
   end if;
   dbms_lob.createtemporary(io_clob, true);
   v_offset := 1;
   v_LENGHT := dbms_lob.getlength(p_blob_in);
   for i in 1 .. ceil(v_LENGHT / v_chunk_size) loop
    dbms_lob.read(p_blob_in, v_chunk_size, v_offset, v_buffer_raw);
    v_buffer_raw := utl_encode.base64_encode(v_buffer_raw);
    v_buffer_varchar := utl_raw.cast_to_varchar2(v_buffer_raw);
    dbms_lob.writeappend(io_clob, length(v_buffer_varchar), v_buffer_varchar);
    v_offset := v_offset + v_chunk_size;
   end loop;

   --io_clob := v_clob;
   --v_result := v_clob;
   --dbms_lob.freetemporary(v_clob);
   --return v_result;
  END BLOB_TO_BASE64_NOAPEX;
  
begin
  select blob_content into l_blob
  from blob_test;
  
  BLOB_TO_BASE64_NOAPEX(l_blob,l_clob);
  dbms_output.put_line( dbms_lob.getlength(l_clob) );
end;
/

drop table blob_test;