How to reproduce this effect (iPhone built in camera app)| darken a part of the label base on the color behind?

I want to know how does iPhone built in camera app darken the background of the label when something white is in focus. Photo below to illustrate what I need:

As you can see a part of the label which is above the keyboard is dark while the rest of label is clear. How do I achieve this? Or where can I start?

EDIT

The question consists of 2 parts:

• how to figure out which color is behind the label?
• how to darken only a part of the label?