Would it be possible to create more than one blacklist as an individual (different reasons)?

For this functionality, it's only one list per account. So if an individual wanted multiple lists, they would need to create multiple accounts (potentially linking them in some advertised way if that was desired).

Or to blacklist people for X reason, where followers can opt-in by checking X reason?

I'm assuming that anyone who wants to advertise their list for following purposes will probably provide the criteria by which they add people to their list. The current global lists usually publish such criteria. My guess is that we will see more "specialized" lists as a result of this change. Some of the types of lists I expect to see are:

• account names similar to service accounts such as exchanges
• accounts that have been trying to scam people or publishing links to phishing sites
• accounts that are plagiarizing content
• accounts that are spamming repetitive or informationless posts