When attempting to make a wither skeleton farm, I noticed that blazes were still spawning even though I had mod proofed all exposed nether brick. Turns out, in a nether fortress, all fortress spawning mobs can spawn on up to 4 blocks of soul soil with nether bricks underneath. The seed used was -5516135316006437641, along with a private vanilla smp with a different seed and this can be reproduced by placing soul soil on top of nether bricks. Both nether rack and soul sand stops the spawning and isn't stopped by putting a air block in between the soul soil and nether brick.