title: Extract blue color channel from a RGB image with scikit
body: <p>I'm very new to python and <em>scikit</em> image processing and I've searched a lot, but did't find the answer to my question.</p>
  
  <p>I need to extract the gray scale of the blue channel from the images I use for further threshold.</p>
  
  <p>I've tried the <code>rgb2gray</code>, but I can't figure it out how to specify only the blue channel for the gray scale conversion. </p>
  
  <p>Is there any other way to do this?</p>
  
  <p>I'm clueless.</p>