there is generally plenty of ambient air in the earth's atmosphere

The stuff the Zeppelin/Blimp floats in

so balloons appear to have negative mass, it's actually just the result of having lower density than the air. the upward force balances out with the gravity where the lbs/in^3 figure of its entirety matches that of ambient air. it's exactly the same as how an empty tank underwater floats, and a water filled tank underwater sinks.

or I guess one could say it's the bottom side getting more compressive load from air than the topside, given the observable effect, whatever floats our zep...

Fill it with hydrogen and just squeeze really hard.