Will these black marks slough off after laser hair removal? (photo)

It looks as though some melanin in your skin, either from tanning or your natural skin tone, absorbed excess laser energy at the settings used. This will resolve, but you must visit a dermatologist ASAP to get started on treatments to prevent postinflammatory pigmentary changes like long-lasting light or dark spots. I would not advise anyone to get laser hair removal treatments in a spa or in any facility where you have not been directly evaluated by a board-certified dermatologist experienced and knowledgable in laser physics and laser treatments, who knows best how to deliver an effective treatment without side effects.

Was
this your first laser hair removal session?
Which laser was used during the treatment? Did your legs appear tan
before treatment? It is not good to perform laser hair removal on tan skin. If
you did not have a tan and you do not have darker skin, the laser was not
performed using proper settings. You need to follow up with the office that
performed the procedure to start on topical therapies to reduce post
inflammatory hyperpigmentation. If you
have darker skin, you need to make certain that the laser hair removal is
performed with a longer wavelength laser to reduce adverse effects. At our office, we have the GentleMax laser which houses 2 wavelengths (GentleLase
at 755 nm and GentleYag at 1064 nm). We use the GentleLase for laser hair
removal treatments on lighter skin patients and GentleYag for laser hair
removal treatments on darker skin patients.