In the book "The Digital Negative" by Jeff Schewe, page 209, figure 5.7 the 'Blend If' part the two sliders are split. The values are 0 / 50 and 200 /255.
The text in the next page is "the sharpening will be fully applied between level 50 (on the left) and level 200 (on the right)" and "This tells Photoshop to roll off blending between level 50 and 0 and between level 200 and 255."
Maybe I have got it wrong but isn't the opposite happening? Levels 0-50 and 200-255 will be sharpened.
Or maybe I have missed something?
What I wrote is correct...the blend of the top layer to the bottom layer happens between level 50 & 200, it rolls off (decreases) between 50-0 and 200-255.