Crackling blades increases the damage applied by spark charges per equipped sword.
This node is not useful even when running a spark charge build, because there are no swords that support spark charges. Swords in the game currently lack spell damage and only have a melee crit chance prefix. In a spark charge build using Fragment of the Enigma, the build is looking for more base crit. This makes rune daggers the best choice in the main hand. There is very little to no reason to use a sword instead of a rune dagger.
I suggest changing this node to per equipped dagger. Perhaps even remove the ‘per equipped’ option, since spark charge builds will most likely use Fragment of the Enigma in the offhand anyway.