this is what i was told by a friend who attends NYU currently,
block classes is when you take a particular subject for a shorter amount of time, instead of taking several subjects over a longer period of time.
For example, usually you would take 3 subjects like biochem, anatomy, and say health laws over a semester of 15 weeks.
With blocking, you would take just biochem for 5 weeks, then you take anatomy for 5 weeks, then you take health laws for 5 weeks.
That is the general idea. That is what I was told.