What is the correct manner to make it clear that your passage is paraphrasing a reference, that itself is refering to another reference?
does A1 not deserve credit for bringing this to your attention??


Not really, however it is often beneficial to the reader to include a reference to a review paper, as it may be a good aid to their own understanding, as presumably it was for you. 

e.g. "A good overview of the field can be found in A1".


  ensure that the paraphrasing is made crystal clear.


State it explicitly.

e.g. Author A1 (2017) gives a good review of recent research into ...., they note that in B1 ....