You may be right about Hubris's worldgen, in which case chances are a custom mod would be required to do this. Unless of course one already exists, in which case we're good to go. I've also seen Rivers that are clear of taint, but they dry up as Flux Goo from all the trees around flows in and destroys the water. However, the biome doesn't change no matter how many Tainted nodes are near it, and in fact I've so far found no way to convert Rivers or Beaches to Tainted Lands. Not that I've tried too hard, since I've been busy trying to actually progress in the game.Yeah. I speculate that Minecraft's worldgen function automatically creates a 'Beach' technical biome every time an Ocean borders any other biome. So you can set worldgen to say, "There's only two biomes...Ocean and Tainted Land." And Minecraft will gleefully agree...and then generate Beaches at their intersections, and the Beach overrides the tainted land.
I've also spotted River biomes that are clear of taint, so those must be added in automatically as well, along the 'fault lines' where different biomes would normally have been.
And Hubris then reduces the rate at which Tainted Lands spread, so those technical biomes don't get consumed very quickly.
Blightfall's map is, as you noted, not the product of Minecraft's worldgen, so it doesn't have that happening in it...and in fact, it also has different 'biomes' that are tainted, which is normally not possible. Tainted Land is Tainted Land, if it worldgens. There's no such thing as 'tainted plains,' 'tainted jungle' or other biome variants.
As for the Blightfall map, I'm pretty sure how he did that was by using natural worldgen to make the world, and then overriding ALL biomes with Tainted Lands except for some select areas. He then probably waited a bit in all the sections to make the Taint progress somewhat and make it look the way it does. I feel like if the worldgen for Hubris did that it would be a lot more interesting, though you'd have to code it separately to convert the world to Taint after everything has generated. If Oceans and Rivers are needed to remain untainted that could also be part of the code but invincible beaches and those other technical biomes could all be tainted this way.