It all depends on how far you are willing to push the blocking and how much you are willing to disable so you can block anything.

Signal atm are using domain fronting. (iirc the app will soon test the network conditions before attempting to use domain fronting, but for now it checks the country code of your phone number)

It will open a HTTPS connection to google.com but after the connection is made sends a host request for something.appspot.com In order to block that you need to MITM the connection or block google.com (Not sure if DPI could be used to get the host header never really looked into it personally. I know that SNI Sends the host is part of the handshake so the webserver knows which cert to present you with. Could it be extracted, checked agasinst a list and then have the connection reset preventing connection? Dunno never played with it, but its an idea off the top of my head).

(Now for some mild rambling :-p)

Lets say you can't MITM/DPI s you can just block google then they would have to use another CDN, so you block that one too. How many you going to go though before your citizens get pissed off at you and do something?

But lets say you people really hated GMail anyway and put up with not having Google just so this message app was blocked (and the creators don't just change CDN's) then you just force your people to install your own Root Cert or they don't get any encrypted web traffic. Will people complain or just install the Cert and get their facebook back?

So people switch to using personal networks (bluetooth and WiFi hotspots when in a crowd of people) just jam Cell/2.4ghz/5ghz. Will people complain they can't use their phones?

And it just escalates to the point you need a Doctors note and a permission slip signed by your mum before you are allowed to make a phone call.

All the time who actually want to encrypt their messages use math they can do at a desk away from a computer or phone and just use whatever method the Goverment do allow / they can get away with (Standard SMS but who and when can be got from the telco's, dead drops, IRL meetings) but sacrifice their metadata in the process.