First of all, we should note that it is a little cold-blooded to talk about positive effects of this horrible human tragedy. We should not lose sight of the fact that millions of people died due to the Black Death.
That said, we can say that something good came of it for the survivors. Because the Black Death killed so many people, there was much more demand for the workers and peasants who survived. They were able to get better wages and working conditions and such after the Black Death. This helped to improve their standard of living and it also helped to give them more power over their lives.
So, while the Black Death was really a terrible thing, it did open European society a bit to people who used to be on the bottom. It made it easier for them to get a good wage for their labors and it made it easier for them to gain more control over their lives rather than being tied to a given piece of land.