Dark spots on gums

I bought this toothpaste to brush my dog’s teeth. I haven’t brushed them before I didn’t know I needed to until now and I went to brush hers and she’s almost a year and she’s got dark spots on her gums over a couple teeth. She does have like a black mark on the top of her mouth so I don’t know if this is just like a birthmark or if it’s cavities I have noticed some blood on some chewy sometimes, but I thought it was coming from Cooper because he has a sensitive spot in his mouth that I’m gonna get him looked at four