I have ~5 years of experience using Krita
Current blur brushes, no matter how you tweak them, will be messy and hard to work with.
Krita lacks an effective blur brush. In a way, doesnt even have one. Becouse all current blur brushes are actually smudge or smudge+scatter which is very hard to work with. I needed a way to equalize blur for a very long time, and tried to achieve it, but never succeeded.
How it works:
Short description: brush samples all pixels under it (in a way that is defined by various parameters), calculates the average color and draws with it.
Long description: based on two parameters (strength and tip shape) pixels within the brush will get adjusted to be closer to the average of all the pixels under the brush at the moment. When applying 100% of the effect without moving the brush, and if half of the pixels are black, half white, all pixels turn 50% black 50% white. When moving the brush, it will sample all the pixels under the brush every frame, disregarding what it sampled before. Or maybe not if it was more advanced.
When it comes to strength and tip shape, that will define how much the pixels will affect the average color based on how far from the center it is. Lets say the tip is very smooth (no hard edges), that means that the pixels by the outside will barely have any impact on the average color of the brush
Algorithmically it should be quite simple, at least compared to most other brush engines