First, as a point of semantics, you are talking about colormaps, not colorbars.
I have attempted at one point a framework to allow users to manipulate colormaps, but it is very difficult to make it work in a generalized framework. However, it is "relatively" easier to specifically hack a particular colormap to get the results you need.
import matplotlib.colors as mcolors
cm.Blues._init() # forces it to internally create the '_lut' array
newBlues = mcolors.LinearSegmentedColormap.from_list("newBlues", cm.Blues._lut[cm.Blues.N/3:-3, :-1], 256)
This should create a new colormap by reinterpolating the last 2/3rds of the Blues colordata to a new 256 colors colormap.
I hope this helps!