According to the official wiki (http://minecraft.gamepedia.com/Nether_fortress#Mobs), Nether Fortress mobs should only spawn on Netherbrick (or due to spawners), among other constrains.
However, blazes (and hence likely Wither Skeletons, though I have yet to witness that) will spawn inside enclosed spaces within a fortress with no spawner in the vicinity, even if the floor is a different block such as cobblestone.