The bevel node works by finding the face normals of your mesh and interpolating between in sharp corners. Normals are just vectors that point directly away from the surface of your mesh. Vectors are commonly represented as RGB values. Which is the color you're seeing on your mesh.
This is just on the "all-positive" side of your mesh. Vectors can point in negative directions. But all colors can only be represented by 0 or positive values. The black is just you seeing a negative vector.
You can tell this because the colors come back when you run this into an "Absolute" node to get only positive numbers.
The bevel node is not meant to change the color of your mesh. Make sure it's plugged into the normal output. If you're not getting the beveled edge effect try increasing the radius or making sure your scale is applied with Ctrl+A.