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...
Attach hooks to the bottom of the blimp and send a guy in a heavy sled with hooks on it, with helium balloons attached to it, to the bottom of the blimp. Attach the sled to the hooks on the blimp, then get the guy to pop all the sled's balloons. The blimp will land on the ground gently, if the math is right.
I always thought the socially inept computer enthusiast who can't detect social cues was an anachronism by this point in history but then I started visiting hackernews.
There you go, solved it.