This Mac Screen Blurring App Actually Helped Me Focus
Multitasking is generally inefficient, and Monocle will help you stop.

Study after study shows that humans can't really multitask but we all keep trying to do it anyway. Why is that?
In part, I blame our nice big computer monitors, which can fit lots of open windows at once. I have a habit of leaving a chat window open in the background when I should be focused on my writing, and I bet you do something similar, even if you know—and agree—with the research that suggests this only increases your distractibility. Recently I stumbled upon Monocle, a $4 indie Mac application that seeks to solve this problem.
Monocle blurs everything on your screen except for your current window. The idea is that this will force you to focus on the task at hand, instead of constantly monitoring several open windows. Sure, you could do the same thing by using all of your applications in full screen or by only having one application open at the time, but this approach means you can still switch easily between windows when that's actually useful (as opposed to whenever your attention wanders).
If this sounds a bit abstract, perhaps some screenshots will make things a bit more clear. Here's a cluttered Mac desktop with multiple windows open:

And here's what that same desktop looks like with Monocle running:

The currently open window is clear, as is the menu bar. All of the other windows, not to mention the wallpaper, are blurred. In theory, blurring everything you're not currently working on will make it easier to focus.
If this strikes you as too extreme, Monocle offers a compromise of sorts: an option that blurs only the bottom three quarters of each window. This allows you to note what's open without getting sucked in:

There are a few more features in the settings worth exploring. You can exclude certain applications from triggering the effect, if you want. You can also adjust the graininess of the blurring and set up keyboard shortcuts for turning the feature on and off.
I tested out Monocle for a couple of days, mostly while writing. That, for me, is the primary task I do better at when I am solely focusing on the task at hand. For this use case, I found Monocle super helpful. It's also the only time I really left the blurring turned on.
That's because there are all sorts of tasks that actually require typing in one window while looking at another. When I'm researching, for example, I tend to have two windows open: Obsidian for my notes, and a browser for the information I'm gathering. I constantly need to be looking at both of those windows at once. I think most people have at least some tasks that likewise require multiple windows, and leaving blurring on during tasks like that isn't going to be useful at all.
The good news is, you can toggle the blurring using either a keyboard shortcut or by clicking a menu bar icon. I recommend doing this often—it's a good way to learn the kinds of work where you do (and do not) benefit from having multiple windows open. I also recommend hiding your Mac's menu bar in addition to turning on the app—that blocks out even more things that could disrupt your focus.
If you're curious about Monocle, the app offers a seven-day free trial. Try it out for yourself and see if it's helpful.