if you mean the first initial upwards kick then yeah you might be able to counter that(I don't think it's unblockable)
but if you mean the animation, then no. once it's started it's started. also big heads up, valkenhayn can combo into his astral. that's about as much info as I have on him at least, you might have to ask someone whose played with valk/against valk for more info.
edit: unless you're talking about his distortion drive where he rushes you down with multiple hits(sturm wolf), you might be able to do that can anyone more experience in that matchup confirm or deny? :X (i'd test it but I'm posting from my computer at work right now)