title: add newline after specific lines with sed
body: <p>I have output as:</p>
  
  <pre><code>A: this thing
  B: that thing
  A: 23
  B: 46
  A: negative
  B: positive
  </code></pre>
  
  <p>which I am creating using sed/awk. How can I use sed (I'm thinking sed is the right choice?) to add a newline after every <code>B:</code> line to yield:</p>
  
  <pre><code>A: this thing
  B: that thing
  
  A: 23
  B: 46
  
  A: negative
  B: positive
  </code></pre>