Vallejo still makes GW's version of Mignight blue.
Sorry I don't remember the name of the color, I used it on my 1ksons.
As an alternative you could go ultramarine blue, hit it with babab black. Then depending on your painting level blend or drybrush it up with ultra, then enchanted.
My suggestion would be ultra then black wash, then paint faded levels of blue lightning forks with white streak in the middle on.