SQL> create table t ( b blob );

Table created.

SQL>
SQL> declare
  2    digits raw(1000) := hextoraw(rpad('FF',1000,'FF'));
  3
  4    l_blob blob;
  5  begin
  6    insert into t values ( empty_blob())
  7    returning b into l_blob;
  8
  9    dbms_lob.writeappend(l_blob,length(digits)/2,digits);
 10    dbms_lob.writeappend(l_blob,length(digits)/2,digits);
 11    dbms_lob.writeappend(l_blob,length(digits)/2,digits);
 12    dbms_lob.writeappend(l_blob,length(digits)/2,digits);
 13  end;
 14  /

PL/SQL procedure successfully completed.

SQL>
SQL> select dbms_lob.getlength(b) from t;

DBMS_LOB.GETLENGTH(B)
---------------------
                 2000