Reverse GrayLevel (not same as GrayTones)

I use Blend[{White, Black}, #1] &

Row[{
  bl1 = BarLegend[{GrayLevel, {-0.1, 0.1}}], 
  bl2 = BarLegend[{{"GrayTones", "Reverse"}, {-0.1, 0.1}}], 
  bl3 = BarLegend[{Blend[{White, Black}, 5 # + 0.5] &, {-0.1, 0.1}}],
  bl4 = BarLegend[{Blend[{Black, White}, 5 # + 0.5] &, {-0.1, 0.1}}]
  }]

Row[{
  BarLegend[{{"MonochromeFractalGradient", "Reverse"}, {-0.1, 0.1}}],
  BarLegend[{{White, Black}, {-0.1, 0.1}}],
  BarLegend[{GrayLevel[1 - #] &, {-0.1, 0.1}}, ColorFunctionScaling -> True]
  }]

None of the other answers mention this, so:

1. For a color scheme like "GrayTones" that belongs to the list ColorData["Gradients"], one can modify them so that they have a different range from the default of $[0,1]$. In this case, if the dark color of "GrayTones" (ColorData["GrayTones"][0]) should correspond to 0.1, and the light color of "GrayTones" (ColorData["GrayTones"][1]) should correspond to -0.1, then one should use something like ColorData[{"GrayTones", {0.1, -0.1}}].

2. Functions like GrayLevel[] and Hue[] do not have this functionality. Thus, one has to use the built-in function specifically designed for the task, Rescale[]. Since one wants a mapping from {-0.1, 0.1} to {1, 0} (note the order!), then GrayLevel should be composed with Rescale[x, {-0.1, 0.1}, {1, 0}].

Thus:

Row[{bl1 = BarLegend[{GrayLevel, {-0.1, 0.1}}], 
     bl2 = BarLegend[{{"GrayTones", "Reverse"}, {-0.1, 0.1}}], 
     bl3 = BarLegend[{ColorData[{"GrayTones", {0.1, -0.1}}], {-0.1, 0.1}}],
     bl4 = BarLegend[{ColorData["GrayTones", Rescale[#, {-0.1, 0.1}, {1, 0}]] &,
                      {-0.1, 0.1}}], 
     bl5 = BarLegend[{GrayLevel[Rescale[#, {-0.1, 0.1}, {1, 0}]] &, {-0.1, 0.1}}]}]

and we see that the middle three gradients are equivalent.

---

As a more striking example of the utility of Rescale[]:

Row[{BarLegend[{Hue, {-0.1, 0.1}}], 
     BarLegend[{Hue[Rescale[#, {-0.1, 0.1}, {1, 0}]] &, {-0.1, 0.1}}]}]