This is a very short theory I wanted to put out there when I thought of it. There is a scene in Warbreaker where Nightblood is at the bottom of the ocean and is only brought up by communicating with a fisherman with a net. It is made very clear that this fisherman is slightly unhinged, as he talks about how the ocean sometimes talks to him and tells him where to fish. We also see in Secret History (If I remember correctly) that Kelsier with Preservation's power could only influence and speak to people who were at least a little insane. I don't remember where I saw this, but I think that was because it means they have cracks in their soul which allows him to just barely get through. We don't know if their is a limit to how close Nightblood has to be to speak to someone, but we can assume that it has some kind of limit, considering how often it might be useful to talk to someone miles away. My theory is that when someone is insane, it allows Nightblood to speak to them from farther away. I don't know what this would imply, but it might show him making a Connection to insane people easier, assuming that's how he does his telepathy. I fully expect this theory to be torn apart, so please feel free!
I've been reading a series called Cradle that is a little less well known, but I have been loving the series. The basic idea is that everyone has a soul that let's them do magic, and when they turn 8 they find out what type of magic they can do. But it doesn't work for this one kid, so he is unsouled. He then has to try to do magic anyway. There are 4 books in the series, with a fifth to come at some point.