Hi Yann, welcome aboard.
A CT will not necessarily detect an outage, unless used on a circuit that always draws current. The CT will only indicate the level of current and from that the power is determined, a circuit that is switched off but still live will appear the same as a circuit during a power outage ie 0 amps 0 power.
It is the voltage that you need to detect for recognising an outage. The emonTx does have an AC detection circuit, so you could sense a single AC circuit, although you are limited to those circuits that have sockets to accomadate your wall wart unless you use another sensor style (eg hardwired component transformer.). With the emonTx, you then have to look at an alternative battery or UPS 5v power supply because it gets it’s power from the AC:AC adatpter usually. So when the circuit in question is not powered, the emontx will not be able to report that unless there is an alternative power source.
There have been various other ways of sensing if a circuit is simply on or off here on the forums (mainly the archived forums IIRC), my favourite is simply to use a mains neon and a light sensor taped together, this gives you a level of isolation, mandatory when sensing mains AC.