Hello Myriam,
That looks like black mold on the leaves. I forget what it is called when it attacks plants, but it should be easy to get rid of.If it is reoccuring, you have a contamination point you need to find. I used a cloth with diluted chlorine bleach to dab the surface of the soil of my plant (chlorine evaporates, so it will not kill the plant if you do not drench it) Cut the blackened damaged portions of the leaves away.
