At least one Modifier will always apply when spending Threads. Spending more Threads improves the chances of multiple Modifiers applying.
Blight Modifiers which can be applied by Threads:

Blight Pump and Towers will spawn friendly Pustules which provide powerful Auras

Up to 2 lanes will be Mycotic

The Blight Pump will spawn a miniboss when successfully defended, with more and better rewards the more Blight enemies defeated in the encounter

Blight towers will have no cost to construct and upgrade

Monsters spawned by one lane of the Blight encounter will be replaced with rolling currency Monsters

Monsters spawned by one lane of the Blight encounter will be replaced by Rogue Exiles, each of which will always drop a Unique item

Blight Pump will be replaced by a slumbering Pantheon God, who will awaken when successfully defended. This God will have more and better drops the more Blight Enemies slain during the encounter, and at least one Rare monster in the Blight encounter will be Pantheon-Touched.