Digital Light & Color

I created a precise Gaussian blur transform with a blur mask in order to just blur the background.
The mask is white for the foreground subject, and black for the background.
I wanted to blur just the background (black). I tried the blur mask menu item "reverse black and white sliders" - this had no effect in the first instance. There were no sliders shown. I tried again, sliders were displayed, but the reversal had no effect.

The issue is probably due to the fact that I should have used an amount mask.

The Reverse Black and White Sliders menu item is not supposed to be displayed in this situation. You need to re-open the mask dialog box if necessary and then click the button at the top of mask dialog box that inverts the mask.

I am looking into why the extra menu item was displayed.

Normally, as you point out, for this simple case you could just use an Amount mask and leave Use Amount Mask as Blur Mask checked.