What's the black gunk in my drain and how do I prevent it from forming?

Hey all,

It's been 6 weeks since I flushed out my bathroom sink with baking soda and boiling water. That made everything shiny and the water flowed just fine. Now, it's got a bunch of black goo that looks a lot like grease on the drain and stopper.

Any idea what causes this stuff and how I can prevent it from building up?

Apologies for the out-of-focus pictures.