Consider a soap bubble and one can see that the colors vary across its surface and even change with time. This is because the soap film varies in thickness. Red light is canceled out where the soap film is thick, making the bubble appear bluish green. Green light is canceled out where the soap film is thinner making the bubble appear violet. As the bubble begins to evaporate, the soap film becomes thinner and all the colors will disappear. The bubble or soap film will be totally transparent and be ready to pop. For a more in-depth look at the physics of light waves and the concept of interference look at the link below.
The answer is: As the film becomes much thinner than the wavelength of blue light, the film appears totally transparent because it reflects no visible light.