Blockade is legal act of war. RU at war with UKR, not NATO, and vice versa. Hence NATO would need casus belli of RU attacking NATO or NATO owned infra to declare blockade (read: declare war on RU).
Russia is already assassinating and sabotaging in NATO countries, which are legal acts of war too.
Russia isn't at war with Ukraine, it's a special military operation. Declaring a little exclusion zone outside all their ports for live-fire naval exercises isn't an act of war either. It'll be temporary, they'll be over by 2028, honest.