Hey guys - figured I'd post this here in case people find it useful or interesting. I pulled off a background blur/glass transparency effect in MonoGame!
If you don't know what I mean, take this macOS screenshot as an example.
I did it without the use of render targets and optimized it as best I could. It works really well in my GUI toolkit managing to pull off a stable 60 FPS (though I only tested with one background blur on-screen, your mileage may vary). It shouldn't be hard to pull the code out of my game/engine and implement it into your own. The effect is similar to Unreal Engine 4's GUI element going by the same name.
I wrote a blog article on how I did it, and the code is also linked in that article. https://bitphoenixsoftware.com/2019/06/11/implementing-a-backgroundd-blur-in-monogame/