The Blacktip design uses two Blue Robotics T100 thrusters. The thrusters are mount to a base plate, as seen in the picture here. That plat in turn mounts to the hull, via two bolts that protrude from the thruster mount.
Using this additional intermediary piece enables the position and orientation of the thrusters with respect to the hull to be easily adjusted, without requiring modifications to the hull. The design choice also had the unintended consequence of easing future upgrades. Blue Robotics has discontinued the T100 thrusters since we made our original purchase – so future designs with either use the T200, or more likely own custom thruster.