Two days ago I went to a salon and had some highlights and lowlights which were supposed to look natural. I am naturally a medium blond. The lowlights are way too dark. Some of them have turned dark brown
My stylist says that they will fade in a month. Is there anything I can do in the meantime to lighten them a bit?

You should go back to the salon and ask them to fix your hair. Explain to the salon manager why you are unhappy with the lowlights results, and he/she will appoint one of their hairstylists to do a corrective color. Good luck.