Clarification on "Spreading Rot" threshold Jewel

The text of [[Spreading Rot]] says " With at least 40 Intelligence in Radius, Enemies Hindered by Blight take 25% increased Chaos Damage"

But the text from Blight says "Enemies who aren't already debuffed by Blight are also hindered for a shorter secondary duration, slowing their movement."

From the two texts, it seems like the bonus damage on the threshold jewel would only apply for the initial application of Hinder from blight, which will be at best ~2.5 seconds.

From my experience previously with Blight, it seems like the hinder is re-applied constantly and basically lasts for the secondary duration after you finish channeling.

Does anyone know? Does the bonus damage only apply for the initial second or so of Blight or does the bonus damage continue applying the whole time you are channeling?

Also, does it stack if you use 2 of them, that is, do enemies hindered by Blight take 50% increased chaos damage? (and since it's enemies take, that's basically 50% more right?)