I doubt it was in the blink of an eye, and it wasn't a kunai, it was a hundreds, that he had them throw all over the battlefield... so that he could appear anywhere if he was in danger... He would be able to appear to any single one of those Kunai giving him a huge advantage.... He does have great reflexes...
But with Naruto in Bijuu mode, now I'd say Minato is only second in the Narutoverse
No you are misunderstanding me, he teleported to multiple kunai yes but in his hand he had only one kunai, which means that he cut their throats or stabbed them to kill them.
But think about it, teleporting to each one, killing the person and then teleporting again and doing it over 50 times before anyone can even react? That is insane, no one has that kind of reaction times except for Naruto now.
I got the blink of an eye from the manga where one of the leaf ninja says if you blink you will miss it. It was likely an exaggeration, but not a very big one.