i've tested it.
it is possible to blink while in ulti but its rly hard to use dagger when all the enemies are dead and to continue slashing ur next target.
and i think that u have to use blink in that moment when he is attacking,when u use it when jugger is jumping to other unit u wont be able to blink.
so u need luck and LOT of training and good reflex...
was testing it about 15 mins and managed to use it correctly(killing my main target and blinking to another while ulti still on) about 2-3 times.lots of times when i blinked ulti was stoped...but managed to blink and after that the ulti was still working.
so,blinking while u let ur first target live is not hard,but to kill both of them is rly hard...