Even pre-smartphone landline systems could block (or route direct to VM) numbers with blocked caller ID.

You absolutely can block such numbers in the stock Android phone app; some online sources indicate iOS lacks this basic feature, but I don't know if they are accurate and current.

How do you block calls from numbers with blocked caller ID in the stock Android phone app? I can't find that option.

“block unknown callers” in the call blocking settings.

That doesn't exist in the latest version of the app. If I go to Settings->Call Blocking it's just a button labeled "ADD A NUMBER" plus a list of blocked numbers. The ADD A NUMBER button triggers a popup with an input field that only allows phone numbers.