oracle blob text search

If it is a Word or PDF document, look into Oracle Text.

This is quite possible and easy to do.

Simply use dbms_lob.instr in conjunction with utl_raw.cast_to_raw

So in your case, if t1 is a BLOB the select would look like:

select *
  from table1
 where dbms_lob.instr (t1, -- the blob
                   utl_raw.cast_to_raw ('foo'), -- the search string cast to raw
                   1, -- where to start. i.e. offset
                   1 -- Which occurrance i.e. 1=first
                    ) > 0 -- location of occurrence. Here I don't care.  Just find any
;

If you are storing plain text it should be a CLOB, not a BLOB, and then you can still query using LIKE. A BLOB contains binary data that Oracle doesn't know the structure of, so it cannot search it in this way.

This works for CLOBs of any length (at least on Oracle 12C):

SQL> create table t1 (c clob);

Table created.

SQL> declare
  2     x clob;
  3  begin
  4     for i in 1..100 loop
  5        x := x || rpad('x', 32767, 'x');
  6     end loop;
  7     x := x || 'z';
  8     for i in 1..100 loop
  9        x := x || rpad('x', 32767, 'x');
 10     end loop;
 11     insert into t1 values (x);
 12  end;
 13  /

PL/SQL procedure successfully completed.

SQL> select dbms_Lob.getlength(c) from t1 where c like '%z%';

DBMS_LOB.GETLENGTH(C)
---------------------
              6553401

Note that there is only one 'z' in that 6,554,401 byte CLOB - right in the middle of it:

SQL> select instr(c, 'z') from t1;

INSTR(C,'Z')
------------
     3276701