Stolen iPhone, Find my iPhone always says "Offline", how can I track it?

😟😢😢😟 I lost it yesterday night, when I phoned it yesterday it rang, but Find my iPhone says "Oflline". Is there any way to track it? Should I blacklist it? And also: could you explain me better what are those IMEI and Blacklist things? (easy English please, I'm Italian and I wanna understand ... thanx a lot)