It will blow up if it is stored as a liquid. And those who use lots of it, store it as a liquid in cooled tanks. If those tanks hit higher temperatures they explode.
The disaster protocol requires that the tanks contents be slowly released out of doors and the immediate area as well as any low lying areas be evacuated until the gas has dissipated.
No, it expands and ruptures it’s tanks. That does not mean it “blows up”. Chlorine gas in and of itself is absolutely, positively, not flammable.