Since there's a separate shape for every re-legislation of a county's borders, plotting each one with fixed alpha means that the dark areas are those in places where county lines have been drawn and redrawn.
Some interesting patterns emerge. The least re-drawn areas tend to be the Piedmont counties, I think, in the South, which is not especially unexpected; but also New Hampshire, Missouri between the Mississippi and Missouri, and the area southeast of San Francisco Bay.
The most tinkered with are central Indiana, going back to various contested claims in the colonial period; the area around Denver; and most of Montana, which was first claimed in 1803 but wasn't subtantially settled and organized for decades. The imprint of the Texas Annexation and the Louisiana purchases are quite clear; the relatively sparse reorganization of Oklahoma dates back to its period as relatively undisturbed Indian territory;
There are also a number of interesting local features, including: the disputed Ohio-Michigan border that led to the Toledo War; several of the independent towns of Virginia; the impressively undisturbed old counties of the lower Hudson; and a very clear outline of the Mississippi.
Nothing profound here, but it's kind of interesting.
You can see a larger version at http://imgur.com/NrVkSdR