Disclaimer: I don't own Inuyasha
It doesn't matter that he couldn't save her, that Tenseiga spoke in dismal swordtongue to still his movement even as she faded, dissipated- slowly breaking down into the basic element of her essence. It doesn't matter that he accepted it as naturally as a temperature change; though the wind forms little whirlwinds of sakura petals sometimes -sometimes- when he's least expecting it, even then it doesn't matter.
It doesn't matter that Naraku thwarted him; such an ingenious method for extermination that even Heaven's Sword cannot revive Kagura at her most vulnerable moment, had Sesshoumaru so desired it.
This is Sesshoumaru, and thus the entirely wrong perspective to take. Sesshoumaru doesn't care that he couldn't save her. There is no need for Sesshoumaru to concern himself with pointless worries, the truly unattainable.
Sesshoumaru knows what's done is done. There are things he cannot change. But icy as he is, he does not find her death meaningless.
It doesn't matter whether or not Sesshoumaru could have blessed Kagura with life again. What does is that he wanted to, that his hand had even gone for Tenseiga in the first place.
And when it comes to this, that is all Sesshoumaru cares about.