T.J. has a black eye because he got into a fight with Stacey.
The blue-black shine that had so nicely encircled T.J.’s left eye for over a week had almost completely faded by the morning T.J. hopped into the back of the wagon beside Stacey …. (ch 5, p. 102)
The fight with T.J. Avery begins when T.J. steals answers to Mama’s history test and blames it on Stacey. When Mama punishes Stacey in front of the class, he wants revenge. Mr. Morrison sees the fight and can’t decide if he is going to tell their parents, but Stacey decides to confess. He does, but does not get in trouble.
This incident demonstrates that Stacy’s character is sound, but his patience has its limits. He is willing to confess for what he did do, but does not want to take the blame for what he doesn’t do. Instead of complaining, he punishes T.J. himself.