IP whitelisting is used to bypass 2FA for those particular IPs & Blacklisting is used to prevent access to the application from that IP.
In the plugin we have provided a feature that you can also configure a range of IPs. For example if you configure 12.1.35.* then all the IPs from 18.104.22.168 to 22.214.171.124 will get whitelisted.
So now one scenario can happen where you have blacklisted a IP (126.96.36.199) but it is already in the range of the whitelisted IPs(188.8.131.52 to 184.108.40.206). In this case, IP blacklisting will be given more preference over whitelisting. So now plugin will block the access of the Atlassian application if any requests come from this particular IP (220.127.116.11).