Is it possible to write an iOS app or app extension to block an incoming call based on the area code (or some other portion of the incoming call number)?
You're right, it only allows blocking individual numbers. Blocking an entire area code would require blocking 10,000,000 numbers, but that's likely way over the limit.
In my opinion, there should be an API to block numbers by regular expression. That would still adhere to the goals of privacy and performance.
Feel free to request new APIs here: https://bugreport.apple.com/