it's not patina - just dark color chert that is no where else on the piece. Now the outside might not be chert(?) - its a bit porous/grainy looking in a few places. but the rest of the white is smooth.
Part of the beauty of flint is the infinite variations of color, texture and blends of the material. The original knapper probably had no idea of the interior until it broke.
The deep thinking part of your find is that ALL flint started out as some piece of biological material. So what creature millions of years ago had a part that would preserve with such a severe color and texture extreme ?