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 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

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 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