My understanding was blau-gass was unpressurized propane, which is a little more dense than air. But I guess according to that article it's slightly less.
I'd expect that, like a lot of problems zeppelins had in the 1920s, burning hydrogen would be more feasible with modern technology.
I'd also point out that the thing that made hydrogen dangerous in those airships was that the skin was two layers. The inner skin was the gas bags, which were very fragile, and then the rigid structure and the outer skin to protect the fragile bags. This was a problem because hydrogen could accumulate and mix with air between the inner and outer skins. The outer skin also was quite flammable. Nowadays, we can make materials that are both strong enough to serve as an outer skin and impervious enough to serve as a gas bag for lifting gas, so modern airships have only one skin and nowhere for the lifting gas to mix with air inside the structure.
