Welcome Emma and Athena!
I think that is impaction, I occationally see dark spots on stomachs when geckos digest food, pass waste or just organs. But it is very concerning when her entire midsection is a blotch. Even if it isn't impaction, better be safe then sorry. Have you exhibited her eating the substrate?
1.) Remove the Eco Earth immediately and lay down some paper towels. You will need to monitor her poops more closely and prevent her from ingesting any more.
2.) Provide us of a rundown on her entire habitat, the size, current substrate, all hides, temperatures, heat source and what it is controlled by.
3.) Tell us her normal eating schedule, what she eats, what suppliments you use, if she is pooping, what her poops look like, etc.
After you remove the ecoearth, closely moniter the area she poops. You then need to start baths, every night.
Tips For Impaction
•Take a tupperware container or a sink and fill with lukwarm water a little over the belly of the gecko.
•Gently massage down the length of your gecko from throat to vent in order to help get things moving.
•Allow a soak and massage session to last around 30 minutes before returning the gecko to the tank.
•Do NOT feed your gecko any more until they begin to pass the impacted substance. You don't want them anymore backed up than they already are.
•Drop a few drops of olive oil on their lips every other day to act as a laxative.
•MONITOR THE GECKO AT ALL TIMES.
Hope this will help you, hopefully you can get us some information back on the gecko so we can further assist you.