Could black people testify in court during Frederick Douglass's time?

There are two prefaces to the book.  The answer to this
question can be found in the one written by William Lloyd
Garrison.

He does not specifically say if blacks could ever
testify in court.  For example, he does not say if blacks could testify against other
blacks in a court of law.  And he does not say if a black person could testify if, for
example, one white man was suing another.

What he does say
is that no master could be convicted of cruelty based on black testimony because black
people were not allowed to testify against white
people.

As to why not, it's because they're
not really seen as people.