Heihe is remote: the train from Harbin takes around ten hours on regular services, and the journey is long enough that overnight trains are worth considering. Heihe Airport has flights to Harbin and a small number of other cities, which shortens the journey significantly if schedules align. Buses from Harbin take around nine hours.
Most logically, Heihe is visited as part of a longer circuit through northern Heilongjiang rather than as a standalone destination from Harbin. If you are traveling through Mohe and the Beiji Village area, Heihe can be reached by bus or taxi from towns along the Heilongjiang River corridor. The new Blagoveshchensk–Heihe Bridge (2022) provides a road crossing to Russia for those with Russian visas.