I am assuming that when you say "this old blackman", you are referring to the hunter Jane and Ned encounter in Chapter 8.
The hunter, an old black man, calls himself "a friend, not an enemy". He is concerned about the children, and thinks they would be better off if they just go back to the plantation. Although Jane insists that they are going to "make it" to Ohio, the old man can see that she has no concept of how monumentous such an undertaking will be. When Jane says, "We done made it this far, we can make it", he retorts,
"You ain't go'n make nothing...two children tramping round in the swamps by themself, I ain't never heard of nothing like this in all my born days...You can't take care you, how can you take care somebody else?...You can't kill a rabbit, you can't kill a bird...Do you know how to catch a fish?"
The old man sees that Jane is only a child and thinks that she has got no sense. He has a concept of reality that Jane does not, and he is afraid of what will happen to her and Ned if she keeps going on towards what she thinks is Ohio. He tells her,
"No map, no guide, no nothing. Like freedom (is) a place coming to meet (you) half way. Well, it ain't coming to meet you. And it might not be there when you get there, either".
The hunter offers, "Y'all want me lead y'all back where y'all come from?", and when his suggestion is met with resistance, he responds, like an exasperated parent, "What I ought to do is knock y'all out and take y'all on back". The hunter tells Jane to go back to the plantation because he knows that she and Ned will be safer there. He tries to stop them from pursuing their quest because he cares about them (Chapter 8).