Joker said:
How big is it?
Is it on such a scale that not even the gods can comprehend it?
Will it ever tip in favor of one of the other side?
Which major players have a vested interest in seeing the Blood War continue?
What are some good sources for information about the Blood War?
Can it ever be influenced in any major way?
Discuss and enlighten.
Hellbound: The Blood War is a beautiful book/box set, and it'll answer most of your questions.
But to echo what has already been said by others, the Blood War is one of the primary concerns across the entirety of the Outer Planes. Literally every outsider race has a vested interest in its course and any ultimate outcome.
The Tanar'ri and Baatezu slaughter one another by the day in numbers per individual battle that dwarf the population of entire prime material worlds; the Yugoloths hurl their lesser kind into that eternal meatgrinder without a care while their greater kind see themselves as the architects of the War itself; the Slaadi seek Xaos and nothing more but have a limited role largely because they're random and unfocused; the Modrons have an army devoted to aiding the side of Law; the Rilmani seek to keep the War eternally balanced in a perpetual stalemate; and the celestials don't know what to do because they can't agree with one another on how to act and what outcome they want (the Archons and Eladrin cannot fathom a victory by the Tanar'ri or Baatezu respectively, and the Guardinals do their best to make sure this Law/Chaos split among the upper planes doesn't ever mirror that the fiends...).
The celestials haven't had a major role in the Blood War since very early on. The Archons once attempted to fight the fiends, and it ended in such catastrophic and onesided bloodshed that none of the celestials have ever since tried to overtly take part (the archons only presented the fiends with a larger target and made them temporarily ignore one another in favor of them).
Gods don't interfere in the Blood War for the most part, because they're haunted by a similar incident that happened during the early years of the War. The cause is unknown, but one deity was obliterated and every other god taking an active role at the time had their divine essence begin to tatter and fray at the edges, threatening them with the same till they abruptly withdrew their influence. The fiends didn't apparently appreciate deific meddling, though it might have been some base feature of the multiverse itself, or the actions of the eldest fiends, or perhaps the wholesale slaughter of mortal worshippers (on a world spanning scorched earth scale). And frankly, beyond the uncertain nature of that past mystery, outside of that threat of unknown cause, it's an honest threat to many gods to stick their noses in the fiends' business now, because entire worlds have in the past been overrun and reduced to smoking balls of cinders spinning in the void of the prime, and a deity is soon to become prime real estate for githyanki if their worshippers were at the business end of such a spillover.
It can be influenced, if in subtle ways. The Rilmani, the Yugoloths, and even some of the celestials manage to do that, each with their own specific aims and reasons but all of them ultimately keeping the War grinding onwards. As for mortals influencing it... not so easily (though the events of the module 'Squaring the Circle' in the Hellbound box had the potential to do so). Long term or radical change would probably need to be something involving the fiends themselves, or a sudden and major (and cathartic) change in the status quo among the celestials.
