> It is unusual for a police officer to incriminate himself.

I don't know how the system works, but from what I have learnt from TV & movies, I guess took the deal to confess, in exchange for a lighter sentence.

> If Hicks hadn't incriminated himself, it sounds like there was no evidence to support Blacks claims.

We already know Black was locked up, bruised, and who his arresting officers were. I seriously doubt if Hicks testified out of the goodness of his heart.