Dark (damp) spots on concrete basement floor
I have had issues with small springs under slabs, yes the dark spot is moisture. The tough thing if it is always there most sealers need the pad to be dry to seal properly. I have had really good luck on several of my own homes with 2 part epoxy applying in late summer. The worst place I would pull gallons of water out of my dehumidifier, once coated with 2 part epoxy paint their was hardly any moisture in the air being removed. The concrete has a high ph so mold is usually not a problem but in an older home the ph may be low or ~8 (when fresh 12-15). But mold in not usually a problem on a clean pad.