Nekron is a demon, but not one in Hell. And his noticing us is, once again, the Oans' fault. (They never make anything better, do they? They just kick problems down the road) Wikipedia summarizes it thus, and this is how I remember it as well:
"Nekron is the ruler of a region adjoining Hell, and which also seems to border on Limbo and Purgatory. It is where the souls of the dead await passage to their final residences in either the Silver City, or the Hell. Nekron draws his power from the souls and spirits of all who have ever died.
When Krona's banished energy form somehow reached Nekron’s dimension--due to a paradox of an immortal now being in the Realm of the Dead--a rift opened between the dimensions, allowing Nekron to see the universe of the living. Desiring it, but being too large to pass through the gap, Nekron instead recreated Krona as an undead being of enormous power. Given an army of similarly-restored spirits, Nekron sent Krona to kill the Guardians of the Universe, so the rift would open more. Krona accepted because part of Nekron's plan involved collapsing the universe so as to recreate it according to his own desires, which would give Krona the chance to see a universe being born."
He was mainly the plot engine for the TALES OF THE GREEN LANTERN CORPS miniseries by Barr & Staton(which also introduced Arisia)--which was mostly a "secret origins" for the whole corps, with this as the thread--as well as Arisia being a rookie--that was providing a reason for all the explanations of the corps' history. Nekron's thing is pretty much as you see above: the absorbing of souls allows him to get into our world. (Wow, I'm amazed how much I can remember from when I was 11.)
So yes, there is now no doubt in my mind Blackest Night is the doing of Nekron, because the Black Lanterns do exactly what Nekron needs. And the particular animus against the Green Lanterns, who stopped him the last time(I forgot how) only strengthens that.
Interestingly, most of the risen Lanterns we've seen prominently fought Nekron.
Also, a little of this may sound familiar:
"Kyle Rayner briefly encountered Nekron in Green Lantern Annual #7, in which Nekron nearly reentered our universe and briefly resurrected the entirety of the deceased Green Lantern Corps members with rings powered by death. Kyle was able to push Nekron back into his dimension and re-seal the rift."
"He has displayed the ability to raise the dead, kill with a touch, fire bolts of black lightning and grow without limit."