Heat the air with direct solar to give it lift. Make it clear on the outside and have something black on the inside to absorb more heat. For bonus points, let it be deployable during flight...fly drone into position in plane mode, pop the blimp, inflating it with air from the forward travel inertia, heat it once with a small rocket motor or something, then let the sun keep it hot enough to float. When done, eject the balloon and fly back. I'm sure there are lots of details around why they don't do this.