Bellibolt has a decent special attack stat and Chilling Water is a super-effective attack that does good damage against Blaziken. On the other hand, Eerie Impulse doesn't deal any damage.
Blaziken is a Attacker not a Special Attacker which makes it useless to decrease its Special Attack with Eerie Impulse, on the contrary, decreasing its Attack will increase Bellibolts survivability.
