Insofar as the Black Death can be said to have been beneficial to one class or another, it was beneficial to those peasants who survived it. The major reason for this was that so many people were killed by the plague. The deaths of so many peasants meant that those peasants who did not die were in a stronger position to bargain with those who owned the land. The landowners needed people to work their land. Therefore, they had to compete to some extent for peasant labor. This led to a situation where life became easier for peasants as landowners had to treat them better in order to get them to stay and work.
The peasants and physical labourers that survived the black death benefitted greatly. Since so many people died, labourers were a rarity, and therefore they were paid better wages, and generally benefitted from the plague.