Proper fork length for a bike with 5.5 inches of rear shock
The different lengths of suspension will affect the geometry by giving your bike a steeper (or not) head angle. Too much travel might make it tougher to pedal as well, depending on the fork's settings. A too long/short fork (compared to what it was designed to use) might put some unnecessary stress on the tubes and for that reason forks outside of a certain range void warranties. 

The blur is designed for a 5.5 in / 140 mm fork. I'd suggest going to a local Santa Cruz dealer (if such a thing exists where you live) and ask them if 120 mm is fine too.