What is the name of the "Things are more visible once you learn about them" phenomenon?
The scenario you describe is sometimes called the Baader-Meinhof Phenomenon.


  Baader-Meinhof is the phenomenon where one stumbles upon some obscure
  piece of information—often an unfamiliar word or name—and soon
  afterwards encounters the same subject again, often repeatedly.


The more scientifically accepted name for this cognitive bias seems to be 'frequency illusion'.


  The illusion in which a word, a name, or other thing that has recently
  come to one's attention suddenly seems to appear with improbable
  frequency shortly afterwards[.]


As to why it occurs, this article seems to state a combination of selective attention (or attentional control) and confirmation bias:


  One, your brain seems to be excited by the fact that you've learned
  something new, and selective attention occurs. Your brain
  subconsciously thinks, "Hey, that's awesome! I'm going to look for
  that thing without actually thinking about it." So now that you're
  looking for it, you find it. To make it all the more powerful,
  confirmation bias occurs after seeing it even once or twice. In other
  words, you start agreeing with yourself that, yup, you're definitely
  seeing it more