SQL> create table tbl(b blob)
  2  /

Table created.

SQL> declare
  2      v_blob blob := utl_raw.cast_to_raw(lpad('9',1000,'9'));
  3  begin
  4      for v_i in 1..20 loop
  5        dbms_lob.append(v_blob,v_blob);
  6      end loop;
  7      insert into tbl values(v_blob);
  8  end;
  9  /

PL/SQL procedure successfully completed.

SQL> select  length(b) length_b,
  2          dbms_lob.getlength(b) getlength_b
  3    from  tbl
  4  /

  LENGTH_B GETLENGTH_B
---------- -----------
1048576000  1048576000

SQL>