When I own 1 planet in a system while a rival empire holds another planet in the same system, and we both have starships in the system, I found that sometimes my planet is considered "blockaded" while sometimes it isn't. The only way to 'break the blockade' is to attack the enemy planet.
I expect to see 3 types of combats being possible:
1) They attack me, my planetary defenses fight alongside my ships.
2) I attack them, their planetary defenses fight alongside their ships.
3) 'Neutral engagement' - our fleets fight each either without the planetary defenses of either being employed.
I'd expect a blockade to be an option you can impose only if you're willing to expose yourself to engagement of type 3, while currently the only way to break the blockade is using attack of type 2. Maybe there can be an interface element should allowing a player to choose if their fleet engages in a blockade, and then one allowing the other player to break it in type 3 combat.
example: game_414, system Seralta.