Nope
No text specificly stating your target.
"You can use this stratagem just before a friendly DEATH GUARD unit shoots or fires overwatch; when resolving the units shots, every model in the unit that is equipped wit ha blight grenade can throw one, instead of only one model in the entire unit being able to do so."
So yes, You can split targets. I run my Plague marines in squads of seven (you know.. fluff) 7d6 grenade hits usually evens out around the 20 odd dice for hit-rolling. My Favorite thing is to keep a Biologust Putrifyer near my squads. S4, D2 & 6+ to wound = a
MW is VERY dangerous for just one
CP.