Response to comments about mains power as per @chrome1000. I live in Australia which uses 240VAC. I very recently bought several Sonoff switches, which switch the mains live wire directly. I have been messing around with wiring things for for 240 VAC for 60 years! There is no way I would try to switch a mains line with these - the terminals are just too close together. They do not meet safety standards. I have met this problem with Chinese products before - a lot of AC-DC converters, for example, are rated for 110V/240V. May be just OK for 110V, but not 240V. Wired carefully, they may be OK for a short while, but with time you will get a , flash, a loud bang and smoke! Sonoff's should be used to switch something like 12V or 24V which in turn switches a properly rated 240V relay.
Having made that comment, as an aged newbie, maybe someone can help with a more general query about Blynk projects. I have been comfortable for years with VB6, but have not programmed usefully in C. My task is to operate a remote heating switch using a mobile phone app. At the switch, there is 3G signal, but no wifi. Thus I bought the Sonoff G1 switch which purportedly can be operated by the 3G link dialed up from the remote smartphone. My phone runs Android 4.4.4. I have spent frustrating days trying to get the app EWeLink registered and have almost given up. That's why I took a look at Blynk. Can Blynk do that? I don't want fancy voice activation - just to press a button on a phone app from 10 km away. Or can someone suggest an alternate app to EWeLink or even if they have managed to get that running on an older Android phone with Kitkat or lower? Hope to hear from you.