What ColorFunction does ContourPlot use by default --- that is, what does "Automatic" mean as the value for that option? The "Density and Contour Plots" tutorial says:
By default, the color ranges from black to white through intermediate shades of blue as the value of the function increases.
And the default gradient does have blue at the dark end; but it goes through something like orange on the way toward white.
Surprisingly, the default does not seem to be among the 51 gradients listed by ColorData["Gradients"].