If you are happy with the way blacklisting with the Asterisk database works, how about a shell script that loads all of the entries in your blacklist file?
Other alternatives involve modifying your dial plan. If you are comfortable with that then you can consider alternatives like an AGI that reads your text file and sets a channel variable.
If you need more ‘immediacy’ maybe storing the blacklist in a ‘real’
database is more appropriate — especially if you want the users to maintain their own black list via a web page.