BlackScreens is an Open Source overlay for your screen that provides adjustable opacity.

This little app is simple but efficient, giving you a way to help protect your eyes from strain. Not only that, but it can also allow you to totally obscure your screen quickly. BlackScreens resided in the systray and can be called up with a single click. The interface is straightforward; it allows you to adjust the opacity of the black overlay, enable/disable click-through and hide the app's overlay text. To turn on/off, all that is required is keying Alt then 1. Simple. You also have the option to set a screen overlay image to be used.

BlackScreens is nicely done and kept simple to allow for day-to-day usability for ergonomic reasons or for quick obscuring your screen when needed - like in a public setting.

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Enable Night Light Mode in Windows 10/11 to Reduce Eye Strain