The Athearn Blue Box locos are not DCC friendly and may require modification to several areas of the loco for a successful installation. Evaluate the loco first by checking for shorts / continuity in several places you first before making a decision.
a) check for continuity / isolate the motor from the frame if needed
b) check for continuity / isolate the couplers from the frame if needed
c) check for continuity / replace the wheels if needed
d) load test the existing motor AND do a stall test! You might need to replace the motor with a low-amp can motor. load cannot exceed 1 amp.
e) hard wire the electrical pickups to the DCC decoder.
The easiest way to isolate the motor from the frame is to reverse the clips that hold the end bearings to the center section. The "bottom" clip has two "fingers" designed to provide power pick up between the frame and the motor. If you put that clip on top and use the top clip on the bottom you now have a pair of convenient tabs to solder wires from the decoder to the motor. You can then solder the other motor wire from the decoder plug to the end of the clip now on the bottom and mount the motor with double sided foam tape to complete the isolation.
example you tube video install https://www.youtube.com/watch?v=VKF7U3ccnco
do a search on you tube for your specific loco
If you decide you want to proceed with the installation the next step is the check the available decoder space in the loco and decide what type of decoder you want to install.