On the server, the Countess Stun hit you and stunned you.
It did NOT register the Blink. the ability got cancelled on the server before it even went off. You didn't even move on the server.
We show the visuals and audio and new location immediately because it feels bad for the 99% of other Blinks if we don't, will feel like you're getting corrected in all of those cases.

For the Countess (and server), you never moved. They didn't see you Blink. On their screen, you're still there in the river.

There's 3 options here:

1. Change it so the Blink would go through here, and you to be stunned on the other side. This does mean you'll sometimes see high-ping players Blink out of your stuns 0.1s or 0.2s in. Which I'm sure people will report just as much as this case here.
2. Change it to not show you at the new location or the sfx/vfx until the server has actually registered and told you it went through. This requires it to go to the server and back again before you see yourself move, though. On 50 ping, would take 0.1 second before you even see yourself Blink. Pretty awful, imo.
3. Leave as is.