Maybe Etrigan won't help them or they need to find a way to keep him on the mortal plane so they just say "@#%^ let's bring Jason back" .Gary indicated it's both roles. So unless it's a flashback, construct, etc. he's being resurrected. Maybe they go to Lady Shiva to ask to use a Lazarus Pit on Blood.
It would totally be a Constantine thing to do to revive Jason to control Etrigan and Jason then hates Constantine for bringing him back from the dead just to be foisted on Etrigan again...