It seemed to me the Blood War was only ever a handwave to explain why the endless forces of extraplanar evil didn't conquer the Prime. I suppose given the changes to outsiders in 4e (in that it's supposed to be much harder for them to reach the "real" world), there's no longer a need for it. Though it makes you wonder what those endless hordes actually occupy their time with... which is something I already wondered WRT the non-evil outsiders.
In any case, as it was only flavor applied to the default cosmology, there's nothng stopping you from still using it.