So, if you take a look at Metabuild like the Infinimist you see, that they took the “Flowing Veins” Pact for "Major DoT multiplier.
After a lot of searching and googling about this i have seen more then one entry and opinion that all “placed” effects are in fact no DoT’s and this way not affected by “+x% Damage over time” or “+x% Shadow damage over time”.
Furthermore i have tested it and the damage is not increased of “blighted corpse” or “Blight” at least not in the Tooltip.
So, what am i missing? Why is the whole DoT-Shadow Damage mechanic so unintuitive? Why is the Keypassive so misleading, because it would make totally sense, if you stack Shadow damage over time to increase both mentioned skills and get the bonus on shadowblight on top.
This take a lot of fun out for me 
I have stopped assuming anything in the game is coded correctly.
At this point is just a roll of the dice if a stat helps you or does nothing.
Yeah, the funny thing is that people talking about proofing facts, by showing Numbers out of the game. Who is saying that those damagenumbers are not broken or misleading, too?
Blighted Corpse Explosion is a DoT as it cannot crit in anyway and it procs XFal’s. Flowing veins not changing the tooltip is correct because it’s only against enemies affected by vampiric curse. It’s works like the control glyph “damage vs crowd controlled” which doesn’t modify tooltip in anyway when equip/unequip. Vulnerable damage from the exploit glyph also doesn’t change the tooltip (at least in the test i just did) but the skeletal mage sacrifice bonus 15%x vulnerability damage does modify tooltip. So i’d say it’s a matter of some conditionals showing and others not on the tooltip but the DoT bonus should work vs vamp cursed enemies regardless.