I have gotten that black stuff all over my MBA's bezel. It wipes off the display, with the microfiber cloth, just fine but it seems to stick to the bezel. It appears to be the remnants of the aluminum bezel rubbing against the aluminum trackpad button. It looks just like graphite, and it seems to appear from aluminum rubbing against aluminum.
At one point there was a member who said he used a custom microfiber cloth that completely covers the entire top case from the display when closing, and I said that wasn't necessary and was a gimmick. However, I have since changed my opinion, and I agree it's a great way to keep the MBA clean.
If you need to clean it, use some alcohol and the MBA cleaning cloth and it rubs right off the aluminum with almost zero effort. Just be careful to keep it on the bezel and don't get it on the display itself.