This is an example of what's known as simultaneous contrast. It happens both in cases like yours where you have light dust against dark black plastic versus 'dark' dust against white tissue. It also occurs in colour where you have one colour against another. You can find lots of nice examples and discussion of simultaneous contrast phenomena on the web. Here are a few sites you might find of interest.

http://www.yorku.ca/eye/simcont2.htm
http://www.cut-the-knot.org/Curriculum/Geometry/Contrast.shtml
http://webexhibits.org/colorart/contrast.html