What is this rash on my dog?

[Hurstlandshome]

Dog went to a new groomers and came back with red spots under his armpits - didn't think much of it but they have now turned into these black marks? Any idea what it could be??

Should add, he is not bothered by it and doesn't try to scratch or rub the area. He is also up to date with flea meds etc and certainly doesn't have them.