i havent dabbled with items but i took a quick look at some of those trigerrables

it seems like they consists of two parts
1. is the triggerable to count how much times youve done an action (kill, hit, crit etc.)
this part is what adds X stats to the character

2. is the triggerable to grant the affix, the SUB TYPE determines when the triggerable will trigger, in this case its when you gain a stat (ADD STAT), the X stat used in the stat watcher will determine which stat the triggerable chooses

FLOATCONDITION is what prevents the add stat triggerable from triggering each time you gain +1 of X stat, until its condition is met

as for LEVEL i have no idea, it might be there to determine what level the character can use the triggerable?
now... all of the above is all speculation based on my experience with triggerables, so take it with a  grain of salt and keep experimenting

and yea its possible to have something trigger when you gain, loose or clear a stat (in this case the "blighted" stat) but you have to set the GAME EVENT TYPE to stat to get the proper drop down options