I really see Justice League Dark and Shadowpact as one and the same, with 'Shadowpact' being the name I prefer and 'Justice League Dark' being the name that sells.
I'd even include Sentinels of Magic and the various gatherings from before they ever had an official name (such as Swamp Thing #50).
That is the really sad thing. Shadowpact is just such a better name. Justice League Dark just sound stupid and always has, but it does appeal to the more dumb comic readers that only buy things with familiar sounding names to other stuff they have read.