You sometimes get this in the summer on ivy that's growing underneath trees, particularly lime trees - they drop sticky and the dirt in the air sticks to it. But its not summer, so unless its been there since last year, its hard to say. Can you post a photo and describe more about its location and environment?

Yes Bamboo is right it is pollution which hasn't had a chance to wash off. You can spray it with soapy water to get rid of it.