Hi,
I’m unsure if this is similar to what you’re asking, but @SchrodingerCat created the Water Color Brush SAI that might work. You can also tweak it for it to be to your liking, such as copying the settings from the cotton brush, namely the Scatter and Smudge Length, to this one for a similar blending, and Color Rate for a more opaque color, less sensitive to light pressure and a steep curve on the medium/hard pressure.
I hope this helps!