This is a limit set inside the PMG code mostly for performance reasons and to prevent a single e-mail address from wasting too much memory+cpu for matching their individual blocklists, but also to help users to use their lists more efficiently
the user whitelist and blacklists can match arbitrary shell-globs - so in that case I'd suggest that you go through the entries in the blacklist and:
* delete duplicates
* join entries which are similar by using globs