I am curious where the information that suspected foremilk-hindmilk so-called "imbalance" should be treated with block nursing is coming from.
As explained above, this is not the current recommendation unless baby is gaining faster than average (in some recommendations, MUCH faster than average) and it is clear that overproduction (not just fast letdown or something else) is the root of the issues. Block nursing was in vogue for some time, and in some cases is certainly appropriate. but recently several breastfeeding experts have raised the alarm that this intervention is causing serious issues when used when not needed or not done carefully.
When considering a breastfeeding intervention, imo, it is important to try benign methods first before trying anything that may cause harm. Block nursing certainly may cause harm. And The longer block nursing is done, the more possible harm it might do. So when tried, it can be tried very briefly-even a day or two may make the needed adjustment in production for nursing to be comfortable for mom and baby.
For issues related to overproduction and forceful letdown, some benign interventions that often work but do no harm would be to encourage baby to nurse more often, nurse one side at a time, use a reclined position when nursing, and hand expressing a little milk prior to baby nursing, and to stop any 'extra' pumping (pumping due to separations may of course still be needed.) It is also important to be sure baby is able to latch and nurse well. A poor latch could explain some symptoms blamed on overproduction, and also, if production is lowered and baby is unable to latch and suckle with normal effectiveness, block nursing may create a situation where baby is no longer able to get enough milk.
Also, milk production usually 'normalizes' over time anyway. So sometimes, it is just a matter of waiting a few weeks while taking any or all of the above measures as needed.
These two articles talk about block nursing, when it might be appropriate, and when not, and how to do it. http://cwgenna.com/blockfeeding.html and http://www.nancymohrbacher.com/blog/tag/block-feeding