Location of blocked numbers within iTunes backup
I looked into your question, and I think it might not be possible to find blocked numbers within a backup (or at least not in a recognizable form). I know that it is possible to find these numbers if you have messages synced with iCloud. Through the messages app on a Mac, you can go to preferences (⌘ + ,), and from there view any blocked numbers that are synced. 



I am still looking through finding the blocked numbers in the backups myself, but I am unsure that I will find them there. If I come across more I will edit the answer further. 

I hope this helps and best of luck of finding the blocked numbers through that backup.

(I am new to StackExchange, so I appreciate any feedback that would help me improve my answers or how/when I answer)