It could be that QF is only willing to sell one seat on the BKK-SYD sector at this time, but has more seats for sale for through traffic on LHR-SYD. And I am not sure how the BA codeshare operates for the kangaroo route services, but it may also be that BA has some quota left while QF has sold most of its quota. Codeshare agreements can vary between carriers and routes, with some being for fixed numbers of seats while other agreements may pool seats between the two marketing carriers and have some form of revenue sharing.
On a normal Kangaroo route 744, QF would have blocked 10 seats (11JK, 16ABJK, 23ABJK). So if more have been blocked, its either due to codeshare or through traffic sales constraints.
When EF shows a seat s "blocked" it generally means its blocked and not yet allocated to any passenger. When a blocked seat is allocated (such as an exit row seats to a Platinum FF member), the EF seat map changes to show it is allocated. So its normally safe to assume that a seat showing in EF as blocked has not yet been allocated.
However, I think what you are seeing is the strange seat map that appears when the flight is transferred to airport control for seating a few days before departure. In this case you are seeing some seats that do not exist, which show on EF as blocked. For example, it is common for the upper deck of a 747 to show DEF seats for rows 11-18 as blocked seats. There are no such seats on a 747 as the upper deck is 2x2 configuration. So if you are seeing seats such as 11DEF, 12DEF, 13DEF, 16DEF, 17DEF, 18DEFJK then this is what has happened. They do not exist and so cannot be sold or allocated.
For that flight EF is showing 1 business class seats as for sale on each of the QF and BA flight numbers. There is no way to tell if this means two seats or if they are trying to sell the same seat.