tail -f $log_file | \
grep "is blacklisted" | \
sed -n 's/.*\[\([0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\)\].*/\1/p' | \
while read line 
do
   echo "found $line"
done