In the U.S. and usually in Canada, blowup is one, unhyphenated word when it functions as a noun referring to (1) an explosion (usually figurative), (2) an outburst of temper, (3) an enlargement of a photograph, or (4) a collapse in performance (especially in sports). It also works as an adjective for items that are inflated with air—for example, blowup doll, blowup bed. The two-word form, blow up, is the verb meaning (1) to explode, (2) to inflate, or (3) to lose one’s temper. Blowup never works as a verb. Blown up, blew up, blows up, and blowing up are always two words.
Outside North America, most publishers use the hyphenated blow-up for the noun senses. The hyphenated form is more common in Canada than in the U.S., but Canada is moving toward blowup.
We don’t know whether a Chinese blowup is around the corner or not. [Forbes]
Following a public blowup at the 2007 World Cup, … Solo bounced back at the 2008 Olympics. [Sports Illustrated]
Another section of the farm contains several blowup bouncers and water slides. [New York Daily News (link now dead)]
Blow-up (outside the U.S.)
Instead, Australian consumers and voters have been spooked … by a Greek sovereign debt default or a US debt and dollar blow-up. [The Australian]
Charl Schwartzel, who won the Masters in April after McIlroy’s final-round blow-up, closed solidly for a three-under 69. [Toronto Star]
A student who put a blow-up sex doll in the toilets at his high school faces a potential eight years in jail. [Daily Mail]
The summer blockbuster season always serves as an unwritten invitation for filmmakers to blow up stuff. [Washington Post]
Greece could blow up at any moment. [Telegraph]