I always defer to this chart when discussing alignments.
In general terms, ai'm running under the impression that D&D ethics, if it can even be called that, run on a mix of deontology and virtue ethics, with consequentialism relegated to a distant third. So in general, evil actions done for a good cause would still be evil. However, one evil act does not an evil person make; only if they become a regular habit can that person be described as evil.
In terms of the Blood War specifically, I feel that if you ask enough people in-universe who know enough about it to make an informed judgment, you'd get a lot of answers of either "it's evil, but it's necessary", or "it's a necessary evil, but it's still evil". Nobody (or at least, nobody who isn't evil themselves) denies that both sides of the conflict are different shades of evil, but the existential threat that would come from won side decisively winning and then turning their attention to the rest of the multiverse heavily outweighs any ethical compunctions informed observers may have. And in the Devils' defense, if they're being given a hard time, they can just point to the Wind Dukes of the Plane of Air and blame them for starting this entire mess.