I am learning that the Blend If function is one of the most powerful in Photoshop. This video from Phlearn is a good overview of the function.
I think one of the best uses of the tool is to moderate the effects of Photoshop’s
“clarity” slider. While the various sharpening modes make universal pixel tone adjustments based on differences with their neighbors (sometimes creating the white halos of oversharpening), clarity focuses more on midtones while adjusting for large tonal areas (as I understand).
But while clarity can enhance apparent detail in mid to light areas, it can seem artificial in dark areas. The way to control this is to apply clarity and then use a gray Blend If to remove the effects from the dark areas.
I learned this from Nick Higham (his page references an early version of Photoshop but the principle still applies)
2022 UPDATE: These days I use the masking tool in the sharpening function in Adobe Camera Raw.