Melee turtles and conveyor belts.
This is a pretty much maximised version of the spawner. A 9x12x9 hollow box(You can make it smaller if you want, but at reduced efficiency). The spawner suspended in the middle 9-10 blocks over the bottom to ensure maximum spawn speed.
Turtles drop the loot in the chests above them. XP floods out under them(add Brain in jars if you want).
As long as there is no direct like of sight to the Blazes they wont start flying and will be pushed towards the turtles.
This setup works with most mobs. With mobs only 1 block high just raise the floor or lower the turtles.