I can't remember the details of the Machakan spiders, but in general I reckon the tactic with a bless is to increase things the unit is already good at. So a unit with defense 10 doesn't benefit that much from a W9 bless compared to a unit with defense 16 (which becomes nigh-on invincible).
I do remember thinking that an E9 bless was good with the spiders, because then they get crazy protection. But I didn't think about it for long, so don't trust me on that.