Below is the shell script, to be configured in crontab, which will send mail incase of blocking session observed in the database .
In the mail body it will contain the blocking sessions details also.

1. Prepare the blocker.sql file.[ for blocking sessions more than 10 seconds)

set feed off
set pagesize 200
set lines 299
col event for a31
SELECT
s.inst_id,
s.blocking_session,
s.sid,
s.serial#,
s.seconds_in_wait,
s.event
FROM
gv$session s
WHERE
blocking_session IS NOT NULL and s.seconds_in_wait > 10;

2. Shell script.(/home/oracle/monitor/blocker.sh )

You need to define the ORACLE_HOME,ORACLE_SID respectively.

export ORACLE_HOME=/oracle/app/oracle/product/12.1.0/dbhome_1
export ORACLE_SID=ORCL
export PATH=$ORACLE_HOME/bin:$PATH
logfile=/home/oracle/monitor/block_alert.log
sqlplus -s "/as sysdba" > /dev/null << EOF
spool $logfile
@/home/oracle/monitor/blocker.sql 
spool off 
exit 
EOF 
count=`cat $logfile|wc -l` 
if [ $count -ge 1 ];
 then mailx -s "BLOCKING SESSION REPORTED IN PROD DB ( > 10 SEC) " support@eraazi.com < $logfile
fi

3. configure in crontab( every one minute)

* * * * * /home/oracle/monitor/blocker.sh > /tmp/block.log
blocking SHELL SCRIPT