You have seen white hot metal, no? And metal that isn't even red hot will obviously be black. I find it quite intuitive. To get a quick-glance impression you just think heat and look quickly at it. I find that keeping the data in a large area of the standard hotmetal palette makes it more clear rather than less, but ymmv of course.
I have to say that I don't find it very intuitive, even though I understand the idea behind it. I look at it, and it makes sense, but it doesn't have the visceral impact that greyscale has for me. Does it takes some time to "train" your eyes to HotMetal?