You can create 100 custom lists in any given BloxOne tenant.

Each list can have 50k entries.

A custom list containing just “example.co.uk” also blocks

test.ftp.sip.example.co.uk

However, cache means that you might need to wait a little while for all queries to get blocked.

For comparison, on NIOS RPZ, you would need example.co.uk and *.example.co.uk

If you put a subdomain in an allow list and you put the parent domain in a block list, then the subdomain will resolve if, and only if, the allow list is further up the security policy rules than the block list.