Block booking seats was apparently done in the distant past, maybe as late as the '70s or maybe even '80s, when wide-bodies first appeared, many airlines rushed to buy them, and subsequently had huge amounts of seats they couldn't sell. At that time, fares (and even inflight service offerings) were regulated by IATA which may have dampened demand (I distinctly remember the cheap APEX (advance purchased excursion) fares for LHR-SIN R/T running ~ GBP 350 in the mid/late '70s while LHR-HKG was fixed at GBP 500 (pretty much pick an airline, any airline). A travel agency (by the name of Eupo-air) in the U.K.(or maybe H.K. which was the same thing back then) catering to the HK market used to buy blocks of seats on BA aircraft (rear section of 747s) to resell. Airlines also sold seats that they'd expect to otherwise go empty through what were known as bucket shops or consolidators.

With rise of better distribution (e.g., better distributed reservations and sales) and less-rigid pricing due to air fare deregulation, the need by airlines to do such has largely disappeared, as have these types of fares.

That said, the unsavvy hear about this past history and believe such types of fares still exist and get taken by less-than-reputable OTAs.