If your tenth permanent enters the battlefield and then a permanent leaves the battlefield immediately afterwards (most likely due to the “Legend Rule” or due to being a creature with 0 toughness), you get the city’s blessing before it leaves the battlefield.

A permanent is any object on the battlefield, including tokens and lands. Spells and emblems aren’t permanents.

Once you have the city’s blessing, you have it for the rest of the game, even if you lose control of some or all of your permanents. The city’s blessing isn’t a permanent itself and can’t be removed by any effect.

If you control ten permanents but don’t control a permanent or resolving spell with ascend, you don’t get the city’s blessing. For example, if you control ten permanents, lose control of one, then cast Golden Demise, you won’t have the city’s blessing and the spell will affect creatures you control.

If you have the city’s blessing and choose not to put the target nonland permanent on top of its owner’s library, it’s returned to its owner’s hand.

If you cast a spell with ascend, you don’t get the city’s blessing until it resolves. Players may respond to that spell by trying to change whether you get the city’s blessing.