In the game I was describing I created two ships with guns, put them in a fleet and put them on guard in the system with the outpost, no enemy fleets were present.   The outpost continued to develop and turned into a colony several turns later.  I'm guessing the number of ships or total attack or siege power is being weighed against the growth rate of the outpost in some way to determine whether the outpost is blockaded or not.

nope, it just continues growing at reduced rate if blockaded. Be careful when using scout ships, because sending investigating anoms autobreaks the blockade allowing possibly stacked supply ships in orbit to come through (so pew pew them first).

If it's riftborn it will just grow regardless cause no shipping.

If it is a high food planet, it will do the same.

If it is a low food planet there is a good chance that you will deny that outpost. - meaning the growth rate will be negative and outpost will eventually be lost.

Player can also see the blockade coming and as a precationary measure invest to double the food on the outpost.

But the number of ships you put there won't make a difference really. I've killed a Lumeris outpost with 2 explorer ships. Building more ships won't make a difference unless the enemy is sending ships to stop your blockade.

If the outpost planet has high food, then you may not be able to stop it period even with a solid blockade. So the X factor you're looking for is kind of pointless.

Oh ok I see what you're saying.  So if the outpost planet has over 'x' food then there's nothing you can do to stop it regardless of if you use a single scout with a pea shooter or have it surrounded by an armada of super carriers.  That's a bit disappointing, I guess those outposts are made out of 'indestructium' or something.

It would still be nice to know where that cut-off point is though, ie: how much food does an outpost need before it engages god-mode.  Would be nice to know whether it's futile or not before I bother diverting resources to it