[Python] Colorbar does not match contour graph

I want to make a contour plot with only 2 colors: Blue if the value is below a threshold, red if it's above. I've also added a colorbar for more clarity. Here's the code portion, I have voluntarily simplified it for this post:

from matplotlib import colors, cm
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.tri as tri

np.random.seed(19680801)
npts = 200
ngridx = 100
ngridy = 200
x = np.random.uniform(-2, 2, npts)
y = np.random.uniform(-2, 2, npts)
z = x * np.exp(-x**2 - y**2)

xi = np.linspace(-2.1, 2.1, ngridx)
yi = np.linspace(-2.1, 2.1, ngridy)

triang = tri.Triangulation(x, y)
interpolator = tri.LinearTriInterpolator(triang, z)

Xi, Yi = np.meshgrid(xi, yi)
zi = interpolator(Xi, Yi)

fig = plt.figure()
ax = fig.subplots()

colormapList = ['blue', 'red']
colorThreshold = np.mean(z)

norm = colors.BoundaryNorm([colorThreshold-1e-3, colorThreshold], ncolors = 2, clip = True)
cmap = colors.LinearSegmentedColormap.from_list('name', colormapList, N = 2)

ax.contour(xi, yi, zi, norm = norm, cmap = cmap)

mappable = cm.ScalarMappable(norm = norm, cmap = cmap)

cbar = fig.colorbar(mappable, ax = ax)
cbar.set_ticks([np.min(zi), colorThreshold, np.max(zi)])
cbar.set_ticklabels([np.min(zi), colorThreshold, np.max(zi)])


The issue I have is that the colorbar should show blue below the threshold and red above the threshold , but it shows white and blue below the threshold, then white above the threshold. For some reason, it also shows a 4th unnamed tick when I only ask for 3 ticks. Here's an example:



What am I missing?

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