male rat has purple blotch on genitals

Question
One of my male rats has a purple blotch on one of his testicles - I just noticed it yesterday.  I haven't seen any bleeding from anywhere, but I've noticed that his anus isn't pink like his brother's.  It's dark (maybe dark brown?).  I tried cleaning this area with a damp tissue, but it stayed the way it was.  I'm not sure if this means that he was bleeding from his anus, and it's now dried.  I'm also not sure if the purple blotch needs to immediately be seen by a vet.  He doesn't seem to be in pain, and, other than the blotch and dark anus, he looks and acts normal (eating, drinking, wrestling with his brother, exploring, etc.).  Could it just be a bruise that will go away in a few days?  The way he and his brother play, it wouldn't surprise me.  I'm just not sure if this is a "wait and see" problem or a "go to your vet immediately" thing.  Although I've owned rats before, these are my first males, so it's the first time I've had to deal with something like this.

Here's additional info: I got him and his brother about two weeks ago.  I think they're only about 7 weeks old.  I keep them in a wire cage with a plastic bottom and use Carefresh Ultra bedding.  I can't remember the brand of food I give them, but it's basically lab blocks for rats, with raisins and sometimes cheddar cheese for treats.

Answer
Hi Melissa
What color is your rats fur?  Often this is due to transparency of the skin in that area and also the pigmentation of the rats skin. If you do not notice any inflammation and he is urinating ok, its probably just due to what I said.  If there was marked swelling (you would know, believe me, I have seen testicles swell to be the size of a lemon before)

Is there a way you can snap a picture of it for me? Sounds like an odd request but it would help me know what is going on since I can see him in person to examine him.