Right before a soap bubble pops, a dark patch almost always shows up first — and it looks like a hole, but it isn't. That's the black film, sometimes called the "black spot" or "black patch." It happens because the soap film has thinned out so much — from water draining out and from evaporation — that its thickness drops below the wavelength of visible light. Once that happens, the light reflecting off the two surfaces of the film stops interfering to produce color: instead of rainbow swirls, you see black. This has been studied for centuries, and part of it is even named after Newton, who first noticed it in soap films back in the 1600s.
From "gray" to "Newton black": two stages of thinning
Physicists actually split black films into two types, depending on how much water is still trapped between the two layers of surfactant. A "gray film" still holds tens of nanometers of water between the two monolayers. A true "Newton black film" is far more extreme — only about 4-5 nanometers thick, essentially just the two soap monolayers pressed together with barely enough water to hydrate the counter-ions, with no free water layer left in between. At that scale, the film stops behaving like an ordinary viscous liquid and starts acting almost elastic.
That black patch is a warning sign: the bubble is right at its breaking point. Once the thickness drops below a critical threshold (around 10 nanometers), the Van der Waals forces pulling the two surfaces together are no longer balanced out by the electrical repulsion from the soap's charges. All it takes then is a small disturbance — a thermal capillary wave, a speck of dust, a puff of air — to nucleate a hole that expands and pops the bubble. So while you can't measure this at home, the longer your mix takes to reach that black stage (thanks to more surface viscoelasticity and better fluid rheology), the tougher your bubble will be. That's exactly what the KUANTIKA simulator models: it estimates how much your formula delays this final thinning before the pop.