Blacklist seems to only add last caller to first extension on a device. If a call comes into another extension on the same device, running Blacklist following a call to an additional extension will attempt to add the last caller to the first extension, not the last call to the device (phone).
Blacklist isn’t an Asterisk application. You’re probably using some sort of GUI on top of Asterisk that implements some kind of blacklisting, or you’re talking about a particular vendor’s phone product. Your question is best posed to them.