DE stands for Desktop Environment. Solus provides Budgie, GNOME, Xfce, and yours, Plasma.
I also use Plasma, and I haven't needed to install Blueman in order to use Bluetooth. I suspect that it's only needed in some GNOME-based editions, not Plasma. I know that Blueman is used by Solus' Xfce edition, and possibly others.
Try just going into your System Settings, look for the Bluetooth entry, and enable Bluetooth and pair devices from there. You might also uninstall Blueman, as you won't need it with Plasma. If you need help with that, let me know.
By the way, I use a Bluetooth mouse and a Bluetooth keyboard with my Solus Plasma computer.