Have not noticed blight going further than 3 tiles on version 1.4.3613. My mods are (in order):
Harmony
(The DLCs)
Cleaning Priority (continued)
Mid-Save Saver
Pick Up And Haul
Follow Me!
Research Reinvented
Replace Stuff
Share The Load
Smarter Construction
RimThreaded
Tiny Table
Simple Sidearms
Common Sense
Research Reinvented: Stepping Stones
RandomPlus
[FSF] Vanilla Bionics Expansion
The only mod I could think of that could cause this would be RimThreaded, (it has been causing some other issues as well).