I have reason to think B is true but I don't know the content of B.

This seems to be a crucial disagreement, so we should settle it first. In your example, you said that you trust Sam to be a good judge and an honest reporter of truth. This means, among other things, that you and Sam share a great many beliefs, and that you think Sam makes judgements roughly in the same ways you do.

So, you mostly understand the kinds of inferences Sam draws, and you mostly understand the beliefs that Sam has. If you infer from this that B is true because Sam says that it is, you must be assuming that B isn't so odd belief that Sam has no competence in assessing it. It must be something Sam is familiar enough to be comfortable with. All that said, you've got a lot of beliefs about what B is, without knowing the specifics.

Essentially, your inference that B is true because Sam says that it is, is the belief that though you don't know what B says specifically, B is very likely to either be one of your beliefs already or something that follows straightforwardly from some of your beliefs.

In other words, if you have good reason to think B is true, you immediately good reason to think you know something about the content of B (i.e. that it is or follows from one of your own beliefs). Thinking that B is probably true just is believing you know something about B.

(ETA: I want to add how closely this example resembles your aliens example, both in the set up, and in how (I think) it should be answered. In both cases, we can look at the example more closely and discover that in drawing the conclusion that the aliens are thinking or that B is true, a great deal is assumed. I'm saying that you can either have these assumptions, but then my translation point follows, or you can deny the translation point, but then you can't have the assumptions necessary to set up your examples.)