If you started with a color photo (which we normally would), and you converted that image to black and white, and then you edited your black and white image so it’s better than the default black and white conversion (still with me?) if you press the \ (backslash) key to see your before image it will show you the original color image.
But when we’re tweaking a black and white image, what we might want (well, I always want), is to see a “before” image of the default black and white versus our tweaks, not jumping all the way back to the color image, which is kinda useless. So, how do we get to that?
There are two ways to get around this:

(1) As soon as you convert to black and white, press Command-N (PC: Ctrl-N) to save the conversion as a snapshot. Now you can get back to your B&W original anytime by going to the Snapshots panel and clicking on that snapshot. Or…

(2) After you convert to black and white, press Command-’ (PC: Ctrl-’) to make a virtual copy, and then do your editing on this copy. That way you can use \ to compare your black and white tweaks against the default black and white.
Hope you find that helpful.
P.S. This part is just for members of the KelbyOne Online Training Community- make sure you check out my class on “Cool layout idea in Lightroom.” I’ve gotten a ton of great feedback on it, and in the class I build lots of those multi-photo layouts, so even if you know how to make them, and just need some new ideas or creative inspiration, give this one a ook. Here’s the link. 
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