What constitutes a blizzard in meterological terms?
A blizzard is defined as 


sustained wind or frequent gusts >= 16 m per second (30 kt or 35 mi per hour)
falling / blowing snow, frequently reducing visibility to less than 400 m (0.25 mi)
3 hours or longer in duration


A storm meeting these qualifications is a blizzard.  Note that snowfall is not necessary, a sustained 35 kt wind blowing snow for 3 hours with low visibility is a blizzard.


  A severe weather condition characterized by high winds and reduced visibilities due to falling or blowing snow. 
  
  The U.S. National Weather Service specifies sustained wind or frequent gusts of 16 m per second (30 kt or 35 mi per hour) or greater, accompanied by falling and/or blowing snow, frequently reducing visibility to less than 400 m (0.25 mi) for 3 hours or longer. Earlier definitions also included a condition of low temperatures, on the order of -7°C (20°F) or lower, or -12°C (10°F) or lower (severe blizzard). The name originated in the United States but it is also used in other countries. In the Antarctic the name is given to violent autumnal winds off the ice cap. In southeastern France, the cold north wind with snow is termed blizzard (see also boulbie). 
  Similar storms in Russian Asia are the buran and purga. In popular usage in the United States and in England, the term is often used for any heavy snowstorm accompanied by strong winds.


Source: AMS Glossary: Blizzard