dry stuff on my pregnant cats vagina?

My cat is going to give birth any day now, she is really really round and her nipples are huge but I have noticed that she has some black stuff on her vagina which I am worried might be dried blood? Or it could just be mud. It's been there for a few days, I tried to wipe it off but she cried when I touched it. Other than that she isn't in any pain and has been behaving normally. Do you think I should be worried about it? I thought she was going to pop about a week ago but they still haven't arrived. She is sleeping and eating a lot but I don't think she's in any trouble because she doesn't seem to be in pain. I don't want to take her to the vet and say "look at my cat's vagina!" because it's like a 15 minute walk and she hates the vet. Do you think I should just wait a few more days and see if she goes in to labour?