In Parashat Toldot, Isaac wrongfully blesses Yaakov instead of Esav. In the Halachah IIRC, most things made wrongfully are illegal and are void, like monetary contracts or marriages.
I don't really know what a blessing is, but for people that hold that it is something, a form of a contract or spiritual influence, Halachicly speaking, why a wrongful blessing wouldn't be simply void?