For some time now, I've been thinking about posting something about the epistemic justification of trust. But I haven't done so until now because I didn't know whether there had been any serious discussion of this topic in the field of philosophy - although one could argue that this subject is dealt with by those working on the epistemology of testimony. Even though I haven't searched for any philosophical work in this area yet, I'd like to say a couple of things that might be plausible to at least some extent.
I've always thought that some key skeptical arguments, such as indiscernibility arguments, have important consequences in the case of trust. As the Academic skeptics used to argue, it seems that we can never discern a true impression or appearance (phantasia) from a false one because they may have all the same traits, i.e., there is no trait which can assure us of the truth of the impression. One usually says that it makes sense or it is justified to trust a given person because of a number of reasons, including certain psychological traits of that person and his past behavior. However, in fact, that does not guarantee anything at all. There seems to be an unbridgeable gap between the evidence we have (or might have) and the truth or what is really the case. So, trust is just irrational and, hence, a highly dangerous risk. Trust is just blind faith - I'm aware that "blind faith" may sound completely redundant.
In the course in professional ethics which I teach, I explain an argument which makes use of the so-called prisoner's dilemma, which is a dilemma discussed in metaethics, business ethics, game theory, etc. I think that this dilemma is an excellent example of how people usually (always?) behave and supports the idea that it doesn't make any sense to trust people: you never know what the other person will do, i.e., you never know whether the other person has the genuine intention to respect the "pact" the two of you have made. And so, it is rational for you not to respect it, and since the other person is in the same situation, then it is only rational for him/her not to respect it either. (Those interested in this dilemma can take a look at this entry.)
So, in the end, it seems to me that there is no epistemic justification whatsoever for this kind of blind faith (or any kind of faith for that matter) and that at most there may be some kind of pragmatic justication in the sense that trusting might sometimes work. In addition, it seems that most of us cannot help trusting people, since to all appearances we need to do so, or don't we?