No blue pill is not undefinable, nor was it something that is purely anti redpill both in scope and origin.

Yes, The bluepill subreddit is reactionary to and anti redpill.

Blue-pill was coined by the redpill as a catch all term for generally accepted social beliefs in dating Such as :

  1. Women are less visuals than men
  2. Just be a good person
  3. You will find your person eventually.
  4. The worst thing she can say is no.
  5. If you are alone then there is something wrong with you
  6. Niceguys are the worst. Etc

There are more that you can think off.

They refer to a general consensus.Red pill is(was) anti conventional wisdom.

It's been so long since redpill came about that some of its tenets seem like common sense now.