The 1850 Fugitive Slave Act was ruled constitutional by SCOTUS (Prigg was 1843, I think). Once the government commissioner found that the person in question was a slave, state laws no longer applied.
Sadly the slave catcher could testify that an alleged slave was a slave, but the accused slave could not testify that he wasn’t a slave.
The only defense was for the accused slave to accuse the slave catcher first, then the slave catcher’s testimony would not be permitted, and the slave’s testimony that the slave catcher was a slave would have to be accepted.
Oh, that is right, the black man was guilty because he was black. Nevermind.