How can I find out which users have been banned?

Unless the ban reason is "alt accounts", and you need the list to link bans, the general list of users blocked shouldn't be terribly relevant. Another possibility would be use a channel bot such as Logviewer with the appropriate permissions and moderator action logging --- then you can search the logs of which users were banned.

Is there a way to track bans?

All bans are required to have a ban message by mods. As of yet there is a lack of open source tools to fully track bans, and obviously a lack of support from twitch. Thank fuck the /user command exists.

Why is my ban message blank?

Come to think of it; the reason supplied in the ban message may be blank due to the moderator just clicking "ban" - may be too brief to assess any kind of appeal without reviewing the chat log and talking with the moderator who set the ban, Since in cases of evasion... in theory, this may not always be completely evident even from chat history.