Wonderful info above.
The only comment I have is that the BB needs to see you as leader, boss, person in charge, and therefore not wiling to make the decision to go after your lab, since your lab belongs to YOU, kind of like it would be with a human child. YOU make the decisions, and it is not acceptable for her to issue any type of correction to your lab, especially not in your presence like that (when you were petting the lab). Any corrections will be issued by YOU, and you don't need her "help" in showing the lab her place. We definitely allow our dogs to settle into roles and rank and all that between the two (or three) of them, but we would not allow any kind of tussle or scuffle right next to us like that. No matter what rank they hold between the two of them, they are equal in your eyes, and they must behave as such. She is at an age where she's trying new things, feeling a little bold and teenager like behavior is expected. This is a great time to review obedience and remind her what is and isn't acceptable. I know human children aren't dogs but if you think of your lab as like a small child, perhaps, it gives your "energy" (the mood your BB detects from you) the right tone so the BB knows that the lab is not hers to abuse as she sees fit. I don't think it will be hard at all to communicate this, as long a your energy is confident and clear.
We've been through this plenty of times over the years, as I have friends we visit with old/elderly dogs, small and large, that my boisterous young dogs have wanted to dominate, and we recently had an elderly and terminally ill dane mix that was bigger than your young dane mix, but he needed protection in this way as his health deteriorated.
I'd perform exercises in which you deliberately have her sit and watch while you pet the lab, then reverse the exercise with the lab sitting for you to pet the BB, etc.
Best of luck, and rest assured, you WILL CUDDLE AGAIN!! You just have to get through this little phase.