You can change the color map you are using via the cmap keyword. The color map 'Greys' provides the effect you want. You can find a list of available maps on the scipy website.
import matplotlib.pyplot as plt
import numpy as np
np.random.seed(101)
g = np.floor(np.random.random((100, 100)) + .5)
plt.subplot(211)
plt.imshow(g)
plt.subplot(212)
plt.imshow(g, cmap='Greys', interpolation='nearest')
plt.savefig('blkwht.png')
plt.show()
which results in:
