I have my blynk configured with my local wifi. Recently I noticed out that even when my wifi is turned off on my phone(I still have LTE connection) I can control my blynk application. I have never configured open port, it is simply configured to connect to my local WiFi. Without configuring anything I am able to control it remotely?
We can’t tell you what your internal network is or isn’t doing, only you can.
This is an issue you really should be diagnosing yourself.
First thing I would do is try to telnet your external IP, from an external network on the Blynk ports.
If you get a connection then your ports are forwarded. If you did not forward them yourself, then maybe check your router settings.
And potential security issues aside, be thankful it works 
Many other users keep asking HOW to do that… even though router settings and port forwarding are not a Blynk specific issue.
If you are using cloud servers, yes, than it works. Because both the App and hardware connect to the Cloud and not a local server.