The Blight is, though. And in my understanding, Corprus is a sort of "advanced" Blight.
That's my understanding for the most part as well.
The Blight is a weather phenomenon associated with Vvardenfell's colossal volcano, Red Mountain. Persistent within the Ghostfence -- that is, within the crater and on the volcano's slopes -- and intermittent near the volcano, the Blight is a health-threatening, ash-heavy volcanic cloud. Plants and creatures exposed to the Blight may contract a variety of blight diseases. The blight diseases you get from being outside during ash storms. The sickness comes from Red Mountain; it makes normal creatures into terrible monsters.
Blight storms, or Blight, are more like the carrier for the blight diseases, these are Dagoth-made. I think they begin from the funnel cloud within the crater.
The encyclopedia does not list corprus as a Blight disease, but I agree it is one.
Blight diseases resist common herbal and magical treatments, ... distort the organism's functions and structures.
So corprus can qualify as a blight disease, but the diseases carried on the blight storms aren't man made, the storms are.