Mobs can spawn on top of upside down stairs.  The top of the block is solid/full so it is counted.

[Here is the wiki][1] on the subject:

> the block directly below it must have a solid top surface (opaque, upside down slabs / stairs and others)

The wiki also states that for a blaze specifically:

>the light level must be 11 or darker


  [1]: https://minecraft.gamepedia.com/Spawn#Spawn_conditions