Denti has it right. British Midland for the European shuttles would set the number of business class rows by design and during turn around the 3 - 3 configuration would become 2 -2 on their 737s for as many rows back as they had business class pax.
However, block boarding does not work unless you have a nice computer model of completely equal SLF. As soon as you have frequent fliers who the airline want to load first, family groups with children under 5 years old, a few wheelchairs etc etc it starts getting difficult.
One way that I saw used in the military was to lay out the seats in the gate area in the format of the aircraft seating with the same numbers and seat the arriving pax into their equivalent seats. Then the problem groups could be identified. When it was putting troops onto a trooper it was possible to send all A seats first in order then all E seats in in order then all B seats in and so on. We could get all the pax onto a 737 or Britannia in less than15 minutes