SQL> host mkdir c:\blahblah

SQL> host copy nul c:\blahblah\test.txt

SQL> create directory test_blahblah as 'c:\blahblah';

Directory created.

SQL> create table test_bfile (bf bfile);

Table created.

SQL> insert into test_bfile values(bfilename('TEST_BLAHBLAH', 'test.txt'));

1 row created.

SQL> declare
  2    lob_loc bfile;
  3  begin
  4    select bf into lob_loc from test_bfile;
  5    if dbms_lob.fileexists(lob_loc) = 1 then
  6      dbms_output.put_line('File exists');
  7    else
  8      dbms_output.put_line('File does not exist');
  9    end if;
 10  end;
 11  /


File exists 
                                                               
PL/SQL procedure successfully completed.

SQL> spool off