Which general term is used to denote such organs/tissues as: brain, testis, thymus etc., which are separated from the blood by blood-tissue barriers?
I am not aware of any umbrella term: the barriers for each are not identical, the function and reasons for the barriers are not the same.
Often these organs are referred to as "immune privileged" because one of the important consequences of the separation is that immune responses in each of those organs are different than in the rest of the systemic circulation, even though this is of course not the only significance of blood-tissue barriers. The meaning of the term has also shifted a bit, though, and I still see different definitions in different places (for example, the Wikipedia definition is not the one I would use). As a consequence, it is easy to argue that some like the thymus should not be considered immune privileged, so my umbrella suggestion would be to just avoid umbrella terms.