What is the blue glow around trunks SSJ2? Is it some sort of pre-God Ki that is going to eventually transcend him to the level of a God? And how was he able to even remotely fight black after Goku and his dad were already defeated?
There is no official explanation, but manga #24 has given a very important hint that can possibly be the answer (spoiler)


   Trunks is officially a kaioshin apprentice, he can use kibito technique to heal other people, and since kaioshins have god ki, as kaioshin apprentice he probably has some grade of knowledge on god ki.