plotting – Is the default color for axis label black in `RegionPlot`?

I had no suspect that the default color for axis label and ticks would be black. However, after exporting an image to pdf, it seems that these colors are not black for me. Indeed, by checking the color with mspaint, the RGB value is (102,102,102).

The following is the code used to generate a plot:

cm = 72/2.54;
r = RegionPlot(x^2 + y^2 < 1, {x, -1, 1}, {y, -1, 1}, Frame -> True, 
  PlotRange -> {{-1, 1}, {-1, 1}}, FrameLabel -> {x, y})
Export("test1.pdf", Show(r, ImageSize -> 7 cm))

The following is the part of the resulting plot that I magnified:

enter image description here

It does not seem as the real black. Is it the expected behavior? How can I make the color “true” black, with RGB (255,255,255)?