Meaning of colors when holding alt/option and clicking on black slider

[TomHackettPhotography]

This question applies to ACR, the Camera Raw filter in Photoshop and the Develop module in Lightroom. I am familiar with using the Blacks slider.  When I hold the Alt/Option key and click on the slider, it reveals those areas that are, I suppose, black in the image.  But it rarely shows them as black.  What is the significance of the color it shows?

For instance, I have a photograph where the woman's dress is a royal blue (#023864, or r-2, g-56, b-100).  When I use the Option key with the Blacks slider, it shows up as cyan.  What is the significance of the cyan color? 

[D Fosse · Accepted solution]

It's a clipping indicator. It shows you which channels are clipping where.

It's just a visual aid. Personally I find it of very little usefulness.

[TomHackettPhotography]

Thank you.  That's actually quite interesting and useful to me anyway.  In the case of the royal blue dress that shows up as cyan when I use Option with the Blacks slider, the red channel is solid black.  As with many of these types of questions, once you know the answer it's obvious.

[D Fosse]

Low end clipping behaves the opposite of high end clipping. When the red channel clips, the result is cyan. No red information = cyan. When the blue channels clips, the result is yellow. And so on.