Huh. It sounds like you're proposing an item three blocks wide that gets referenced whenever a block on the same longitude wants to be infected by a nearby block. Which is actually a pretty clean way of doing it.
But doing it like another block state - 'can't be corrupted', like paint or actuators, actually seems more versatile to me. You'd be able to use it for builds - evil grass next to regular grass, feathered stone and ebonstone, etc - or to cordon off horizontal swathes of land as well as vertical ones. But I'm inclined to think that just placing an object and cordoning off a spreading stripe is too easy unless it requires very lategame ingredients - think the DCU (which is equally useful for sectioning off biomes, but in this case, the goal is aesthetics.) It outperforms the Clentaminator significantly and removes all urgency to one of the most difficult and time-consuming parts of the game, albeit one I think isn't that well designed. It especially outperforms when it comes to cordoning off the jungle, where thorns and vines make tunnels much dicier.
Tl;dr, I'd support something that lets an endgame player change the way spreading biomes are warded off, but not something that renders the Clentaminator or the early-hardmode panic obsolete.
