Did the black dogs have any behavioral or energy similarities?
Like the lab - my GSD is very good at ignoring other dogs - she prefers a nice poodle over a larger dog
but is never snappish. However, when a dog of energy, or a "close talker" comes up at her, a lip will go up in warning. That's when I make sure to step in and body block so she won't have to deal with it (because if they ignored her lip, she'd probably tolerate their attention).
Labs, Boxers, bully breeds all fall into her do not want list in general because of their energy and lack of boundaries. If any of those dogs ever can show appropriate (to her) behavior, she's fine with them - I had a lab mix foster who was very smart (probably mixed w/Border Collie) and she made a big deal of approaching nicely, and my GSD was okay with that.
So I would look at the energy and behavior first, see if you can find similarities on that list. Because all that being said, my GSD did not like chocolate labs for a while after one in our obedience class who would climb up our hind ends in healing exercises, and bit my finger trying to steal my dog's treat. For a couple of years, she'd see that color and be like "let's get out of here." She didn't want to wait around to see if it was that one, or if it was going to be polite, she just wanted to avoid.