Ugh, yes a nick can happen *easily*, and it certainly doesn't mean they're a bad groomer... Gawd, if that was the case then I'm a terrible groomer cos I've nicked two dogs now.......
I did a very similar nick on a schnauzer once, using a 10 blade on the sanitary (any blade can nick a dog! some are more likely to than others though), I stood her up on her back legs (holding her front legs) to check it and do a teeny adjustment (I had left a chunk of fluff and wanted to zip it off) and cos she's a lean wee girl she had a wee fold of skin right down the middle of her belly, and *zip* I nicked into that fold! Yeouch, I felt terrible, though the dog never noticed! I was on the phone to the owner within minutes and by taking that responsibility for it we have kept her as a good customer.
At the same time I've had people come to me and tell me their previous groomer nicked their dog and then denied they did it!!!! I've had others come to me who the groomer never told them about a cut or something too, though in those cases I do tell them they should have let their groomer know, as perhaps for whatever reason they hadn't realised it was a cut or whatever....
So yes, it is somewhat of a potential hazard of the job. We work with a lot of very sharp tools, and despite obviously knowing how to sue them and knowing how to avoid nicks or cuts, it can still happen when you least expect it!! I can almost imagine how a nick could not be noticed cos both times I nicked a dog, the dog never reacted!
I think at the very least you should call them and tell them you found the nick on her belly, and see what they then say... I'm not saying you should go back, cos they've stuffed you around before (though they did try to make ammends as best they really can....) but see what they say about the nick before writting them off totally. I know I wonder what on earth happened when a customer never turns up again, and if someone calls to complain about something I did wrong, I'm GLAD, cos at least then at least I have the chance to try fix it for them!!
((hugs)) to your poor wee girl. I hope it's only a small nick and has scabbed over by now, give the groomers a call and see what they say, you can still tell them where to go or that you're not interested in going back to them, but at least they know.