SQL> DESC a1_test;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID                                                 NUMBER(2)
 BLOB_CONTENT                                       BLOB
 BLOB_HASH                                          RAW(40)

SQL> SELECT DISTINCT blob_content FROM a1_test;
SELECT DISTINCT blob_content FROM a1_test
                *
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected - got BLOB

SQL> DECLARE
  2     l_hash   RAW (40);
  3  BEGIN
  4     FOR cur_r IN (SELECT id, blob_content FROM a1_test)
  5     LOOP
  6        l_hash := DBMS_CRYPTO.hash (cur_r.blob_content, DBMS_CRYPTO.hash_sh1);
  7
  8        UPDATE a1_test
  9           SET blob_hash = l_hash
 10         WHERE id = cur_r.id;
 11     END LOOP;
 12  END;
 13  /

PL/SQL procedure successfully completed.

SQL> COL blob_hash FORMAT a40;
SQL>
SQL> SELECT DISTINCT blob_hash FROM a1_test;

BLOB_HASH
----------------------------------------
14D0966C40D7A3FEF653361EC2D6776DB5A4673F
2DBF1AA0F0C68215156AFEB35E5FA42516285434
F4C8AD2C6F67CA585AD17ED70EE8167BCD6C4B68
B144A1FCADF95C98DDA27E537A4D0EF4C8453EC9
27DD799F685856627C0BFCB2B131E66B55D691A4
B77E6C247AD8CBDFCCC59FC66AC148235166ABCF
C0F44FE2DCC3B71238EF44F0E349A5765EB7C8E5
6DA0114E7BDCB0EA0191132DCBB8E27F3D0C25A6

8 rows selected.

SQL>