For the longest time, Linux Mint depended on Blueberry for its Bluetooth background service. With the release of Linux Mint 21, that all changes, as the developers have opted to migrate to Blueman.
One of the primary reasons for this change is that the latest version of gnome-bluetooth (the bluetooth backend for Blueberry) introduced a few changes that broke compatibility with Blueberry. Unfortunately, the Blueberry developer has no desire to see his work used outside of GNOME. Because of this, Blueberry will have trouble with non-GNOME desktops going forward, which is one of the reasons why the Linux Mint team decided to go a different route.
Another reason for this change is the Blueman works better with Bluetooth audio headsets and can connect to a much wider range of devices.
The Linux Mint developers are currently in the process of integrating Blueman with the desktop OS and hope to have it ready for the full release of iteration 21. Currently, there is no official release date set for Linux Mint 21, but (if history is any guide) it should be sometime this summer, 2022.
To read more about this upcoming change, and other interesting bits about the new release, check out this issue of the Linux Mint Monthly Newsletter.