My hamster has a black spot?

I've never noticed it before, but on his side he has a little black spot. Also, his fur is wet around it, and i don't know if it's because he's just walked past his water bottle, and I had only just took him out yesterday. Has he always had it? and im just noticing it because he's not so furry just now because of the wetness, or is it symptoms to something...?