For a double-Black mana casting cost, one target creature and all other creatures with the same name as that creature get -3/-3 until end of turn. Of course, it can hit tokens, which is nice. Often times, Bile Blight allows you to 2-for-1, or even 3-for-1 in some cases... even 4-for-1 if you’re lucky.
Bile Blight is a bit situational, and granted it can hit your creatures if they share the same name, as well. But, in any case, it’s not bad against some decks. It’s definitely been useful in the past, and while it rarely sees much competitive play anymore, it's still a good card!