Expected behavior: Sculk blooming to blocks under and around (at level) the sculk catalyst when mobs die on top of sculk catalyst. 

Observed Behavior: When a mob dies on top of a sculk catalyst, the bloom will not expand to blocks at grade or underneath the catalyst, unless it first starts from above the catalyst. 

Testing: I tested this by dropping skeletons onto the catalysts, and placing blocks at different heights next to the catalyst. 
In the image, this was accomplished by placing the catalyst: above, inside, and below the ice. Only the catalyst placed below the ice bloomed from mob death directly on top of the catalyst.