If I attack a Blob With my own Blob, and it gets to the point where they are considered attacker and defender, I flip over Lost City, pass Priority and no response from opponent. I power up my Blob and pass, my opponent Kabooms my lost City In response to the Power Up. So the Kaboom resolves, and then the Power up occurs, does my blob get +1/+1 or +3/+3.

I'm not clear if the Power up Checks at the time of Payment (the discard, With Lost City still in Play) or when it resolves on the chain (After the Kaboom has resolved and no Lost City in play).

Just Need ta know, ya know...

The Replacement that Lost City generates is done when the Power Up resolves. If the Lost City is not in play at this time(such as the situation you described) the Power Up will ongly give +1/+1

Thanks Mitchell, you're the bomb in Phantoms, Yo !!!!!

That's what we figured. but I wanted to be sure about it.

So in another senario, I flip over Lost City and Pass P. and they decide to Kaboom it then. I can respond buy powering up, getting the +3/+3.

I assume that is true also, it's just a really bad play on their part, they should wait for the power up as in the original scenario I Poated.

Correct

Although we don't usually do this when playing in a game, but technically speaking, if you flipped Lost City and then passed and your opponent also passed, wouldn't that just bump you to attack outcome because of the double pass and you wouldn't be able to power-up anymore?

This happens a lot in games and we just ignore it and say stuff like, "well... I didn't mean to pass or I meant to play this". It's okay in occasional games but could get sticky during PCQs.

While we commonly say if both players pass on an empty chain, they game moves to the next phase or step, it would be more accurate to say, When both players pass on an empty chain and no other intervening actiond, the game moves on.

While fliping over a lost city does not using the chain, it is considered an intervening action and requires both players to pass priority again.