Oh yes, black labs can be aggressive. I have known a couple.
One in particular belonged to my good buddy/cousin and his wife and kids. We were pretty tight, at each other's house a couple times/week. And "Old Jake" recognized me and mine as part of the family. And he was mellow as hell when his family was home.
But if they were not home, I was the only person Jake would let into the yard, let alone the house. Jake came to live with us for about six weeks once. Within a couple weeks I heard stories from people attempting to come to my house. Jake had adopted our house as his own, and nobody was getting close if we were not home.
