There have been many, many brawls between the Blues and Blackhawks. There’ve been more brawls between fans of the two teams than some actual teams have fought each other.
Sometimes, though fights take on a level of mythos that goes past brawl or skirmish. Sometimes they turn into massacres. The Blues and Blackhawks had one of those on March 17th, 1991. The St. Patrick’s Day Massacre was a byproduct of two heated rivals fighting for the top of the Norris Division and the league itself all season long. Something was going to break at some point, and when it did, the whole dam went with it.