Erik Mona said:
Is it established within the D&D lore (Planescape, etc.) that the Blood War ever encompassed more than just fiends?
--Erik
http://www.superunicorn.com/erik
In addition to what Alzrius said:
During the early stages of the war there were non fiend origin deities that got involved in the conflict. Initially the Tanar'ri had various deities of chaos on their side, and for a time they were almost unstoppable. Then the Baatezu convinced more and more deities of law to join on their behalf and it swung back to a stalemate again. But then something happened. Every deity involved began to notice that their divine essence was starting to fray at the edges, to diminish ever so slightly at a constant rate. And then a deity of chaos simply died without obvious reason. Fearing the same happening to them, every deity withdrew from active participation in the Blood War, and it's been that way since.
We don't know why it happened, though its been suggested that one or more of the fiend races didn't appreciate deities fighting in 'their' war and began a wholesale slaughter of mortal worshippers on the prime material. Or it might have been just something about the nature of the planes themselves, nobody knows.
With the celestials and their involvement:
There's evidence to suggest that at some point the Archons and Aasimon/Angels of LG deities were at war with the Ancient Baatorians who predated the creation of the Baatezu. Some of those ancients can be dimly seen, frozen within the glaciers of Cania along with Archons and Aasimon fighting them, and bizarre cities presumed to have been created by that ancient race.
Since the disaster of the Archon and Aasimon crusade into the Blood War in which they took on all three sides at once, only to have them all turn as one on them, the upper planes have taken either a large role or a direct role, but never both. Some covertly supply the fiends with information or weapons, both sides, trying to keep the two opposing sides balanced (the yugoloths don't appreciate this since they see that as their role, and the Blood War as 'their' experiment into the nature of Evil).
The upper planes are just as divided over the law/chaos axis as the fiends are, but they won't go to genocidal war over it. They obviously have differing opinions on which side they'd prefer to win the Blood War if it ever came down to having one side or the other win. The Eladrin can't conceive of a Baatezu victory, and likewise the Archons can't fathom the Abyss winning, and the Guardinals want to see all three sides obliterated, especially the yugoloths.
Guardinals in small groups will descend into the lower planes and slaughter fiends to their hearts content, usually taking heavy losses but not caring. Similar thing with Asuras, but for them it's less organized.
There's also information in 'Hellbound' regarding how the Slaadi, Modrons, Rilmani, Gehreleths/Demodands view the conflict and how they're involved. As well it has information on how deities view it all, how the various planar factions view it all etc.
It's a beautiful book, comprehensive in how it mentions the Blood War affecting every plane, race, etc either directly or indirectly.