Implementación de la función HEXTORAW para dar soporte a CLOBS:

create or replace
procedure hextoblob(
                    src_clob  in       clob,
                    dest_blob in out   nocopy blob
				   )
is
    v_pos    number := 1;
    v_amount number := 1024;
    v_buffer varchar2(1024);
    v_raw    raw(2048);
begin

    while v_pos < dbms_lob.getlength(src_clob)
    loop
        dbms_lob.read(
            lob_loc => src_clob, 
            amount  => v_amount, 
            offset  => v_pos, 
            buffer  => v_buffer);
        
        v_raw := hextoraw(v_buffer);

        dbms_lob.writeappend(
            lob_loc => dest_blob, 
            amount  => utl_raw.length(v_raw), 
            buffer  => v_raw);
        
        v_pos := v_pos + v_amount;
    end loop;
    
end hextoblob;

Para probar el procedimiento podemos ejecutar el siguinte código:

declare
  cl_test   clob := '4041424344';
  bl_result blob;
begin

  dbms_lob.createtemporary(
    lob_loc => bl_result,
    cache   => false,
    dur     => dbms_lob.call);

  hextoblob(
    src_clob  => cl_test,
    dest_blob => bl_result);

  dbms_output.put_line('Result: '||
                        utl_raw.cast_to_varchar2( bl_result ));

  dbms_lob.freetemporary( bl_result );

end;

El resultado debe ser:

Result: @ABCD