What is causing her skin to look like this?

I had to crop it a lot to be able to see this because she is nude, but what is causing her skin to look all black and dirty? Too underexposed maybe? Canon 6D 24-70 @ 25mm f/8 1/125 thanks!

 

I'm viewing this on my mobile (in my sunny balcony) but I don't see 'black and dirty'. Lighting and exposure us a matter of taste. Exposure and highlights/shadows can of course be adusted in PP

 

I do see 2 distracting 'shapes'. Maybe due to the crop. One top-left and the other (blue) in the girl's hair. Please ignore if irrelevant.

I don't see "black and dirty" on my calibrated monitor. Could you circle the offending areas?

I'm not sure what you mght mean.

IMHO her back, where a backpack strap would be running, and rear sides of her arms are suffering from less flattering lighting and overdosed DOF. Your light seems to brush these shadow areas and does a great job sculpting her wrinkles & goose bumps like surface.

Ronbrown, I think you're referring to some of the shadow areas that seem to be devoid of any color? Not sure. Did you play with saturation and do any color adjustments? It doesn't look bad to me. I wouldn't call it black or dirty. But her hair color is possibly influencing and effecting the shadows.