I'm adding a lot of contrast to some images in PS (+100), but I'd like to keep the dark areas, like that, because otherwise they get all black and I lose the details.
Is it possible to do something like that?, like setting a sort of dark threshold.
Try using a Levels adjustment layer. You can alter the darks, mids and lights separately. You can also set the opacity or blending mode for this layer independently to tweak the effect even further.
You need to mask the areas to protect. A simple luminasity mask will do the job.
As you make the adjustments you will see that the dark areas will be protected due to the mask.
Use "Select > Color Range..." set Fuzziness to 200, and then use the dropper to select a neutral tone that is close to the range that you want to adjust.
Alternatively, use the dropper to select the tone range you want to leave alone, and then invert the selection area afterwards.
Optional: Ctrl+H to hide the selection without deselecting it.
Use the levels tool to adjust the mid-point.