If you are using PIL, you can use
trans.convert('L')

which will convert the RGB image into greyscale (more here 80). You can now plot the image using
plt.imshow(trans)

If that gives non-greyscaled colors, you can force the colormap to be black and white using matplotlib’s binary colormap by writing:
plt.imshow(trans, cmap=plt.cm.binary)

Here, I’m assuming type(trans) is <class 'PIL.Image.Image'>.