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.
The DC motors in blue box locos require large amounts of power due to age and the motor design. We have no decoders for this application. My advice is to look for a decoder specifically designed for high current draw applications or Blue Box locos at least 1.5 amps of current capacity. The loco decoder needs to be able to handle 1.5 amps of continuous current draw. If you plan on running more than one of these at the same time or any additional locos, you will need a 5-amp system or a 5-amp booster.