Nearly forgot to answer this one - its for warding off crows.
Crows in Tokyo have been a problem for a long time. When rubbish is thrown out in the morning, you hear them cackling to each other communicating areas with the most rubbish. They then swoop in and make a complete mess.
Not sure how effective these black bags are though. Other methods used for warding off crows are CDs hanging on a string and changing the color of the nets that are used to cover the rubbish.
The net colors deter the crows for a while until they learn that its just a net with a different color - and so the cycle continues.
So far in our neighborhood, we've had green, blue and yellow nets for the rubbish - or "trash" or "garbage" depending on where you live.
Yesterdays text below.
Two stick like objects with black objects hanging from them by a bit of string - can you guess why some Japanese put these outside their houses?