Blight growth speed is determined by the cell type. The cell type affects the speed of all units, and it depends on their track type. One of the types seems to be specialized for blight growth. I've played with it before to see what happens. Cliffs have much different values, hence why it spreads so much slower over them. It's also different depending on whether it's the high side or the low side of the cliff.
Btw, speaking of oh s*** scenarios....