Green leaves absorb blue and red light, which are the colors of light needed for chloroplasts to do photosynthesis.

It appears to be an artifact of the history of the biology on the planet that leaves are not purple, so that they could use more of the sunlight which peaks in the green part of the spectrum. Apparently purple bacteria got there first, using up all the green light, and only purple light was left over so green plants evolved to use purple light.

Of course black leaves would absorb more light, but they would simply absorb most of the light as useless heat. What you really want to do is convert photons into sugar and oxygen molecules. Purple light does that in chloroplasts, so leaves only absorb purple light.

See: Risposta di Jose Pineda a Da una prospettiva evolutiva, perché le foglie sono verdi?

Risposta di Adam Wu a Perché la clorofilla si è evoluta per essere verde invece che nera, che avrebbe assorbito più energia?