I really like the inhumans, but this show really disappointed so far. I am looking forward to the kree stuff though. I really like the kree and we know that the new season of Agents of SHIELD are using the kree. So it is pretty easy to put two and two together. The threat that blackbolt is talking about is the Kree.
One of my favorite Inhuman stories is from 2000, where the kree come to earth to take the inhumans into space and use them as weapons.
I am thinking the kree couldn't sense the inhumans under the dome, but now it is gone the kree can find them and that is why they have returned to earth. Or atleast with all the inhuman awakenings from AOS they have peaked the interest of the Kree. But now that blackbolt is in play I think the kree really want him.
I really think blackbolt doesn't seem to powerful, aside from his voice, he doesn't seem to have super strength and can't fly. I am hoping this is just an oversight that will be corrected, or at the very least he has been holding back. We shall see when he fights the kree.