That would be known as the Halifax Explosion.
Not sure about the song, but there is a song collected by Dr. Helen Creighton which is in the DT out of one of her books.
What the story is....
On Dec 6, 1917, at just after 9AM, two ships, the Mont Blanc and the Imo, met near the middle of Halifax's harbour, on the Dartmouth side. The ammunition ship, the Mont Blanc, had been in the harbour since the 5th and was on her way off to France to deliver her supplies. The Belgian ship apparently ignored standard practice and somehow ploughed into the munition ship. A fire started on the Mont Blanc. The crew abandoned her and she drifted over to the Halifax side.
Crowds gathered as the ships were burning. The Mont Blanc was not flying flags that indicated the cargo was explosive. That would have made her a target as she sailed the Atlantic.
The explosion that resulted was the LARGEST man-made explosion prior to the Atomic Bomb. It killed thousands of people outright and left many thousands injured. As many people were standing in front of windows looking out at the fires, a lot of the injuries were blindness caused by the flying glass. However the bulk of the deaths were due to fires caused when the explosion caused many of the wooden homes in the North End of Halifax to collapse upon themselves. Most homes of the day were heated by coal or wood. Also, the VERY next day there was a blizzard to compound the problems for rescuers.
Check out the various websites, using Halifax Explosion or Her Own War or Shattered City
Anyway on to the Boston Connection....
The city of Boston was among many groups who sent people, supplies, etc to help out. Including many doctors, and medical personnel. One of the reasons was the number of Maritimers who had relatives in the Boston region.
In the 1970s, the then politicos decided that something was needed to thank Boston, so an annual Christmas tree was sent on our behalf. Not quite sure who started it, the Provincial or City officials.
Among the results of the explosion were
1 - Standardization of fire equipment NATIONALLY
2 - Emergency/Disaster planning was created
3 - Hydrostone as a construction material was proven (ie cement)
Check the books and web-sites for further details. Lots of interesting stuff.