What is Black Liquid Sorrow?

BLS is an indicator for how many nerfs your kittens have lived through. The latest change, which added the mechanic, also nerfed compendiums and zebras slightly; it's unclear which one counted here. With the current implementation, you only get one sorrow point even if multiple nerfs have occurred since your last time playing.

BLS doesn't actually have any in-game effect yet, but a guess is that it's been added due to this reddit thread, where it was proposed to give a tiny boost to tear production. We don't know if that is what it will actually end up doing, though.

The current accepted answer, written in 2014, is no longer correct.

BLS is a resource derived from unicorn tears used to build Black Pyramids.

For more details, see the wiki.