I'm no balloon expert, but it seems like they could have absorbed more heat with a translucent top. Then the solar rays would enter the chamber before hitting

I'm no balloon expert, but it seems like they could have absorbed more heat with a translucent top. Then the solar rays would enter the chamber before hitting the black fabric and reflecting back into the chamber.. more frequencies of light, more chances to catch the heat. Like the SHGC effect observed by closing/opening curtains