No, putting black bin bags on a pool does not work. The plastic is too thin and will tear easily due to the pressure of the water it's covering. This means that:
* Sunlight can still penetrate through the bag;
* Bags may rip or float away;
* Any contaminants in the water cannot be blocked from entering your pool. Therefore, this method should not be used as an effective way of blocking sunlight from entering the pool.