I believe what Lodge sells is a "rust eraser" block meant for surface rust, but doesn't appear to be aimed at other issues. Stubborn black stains are typically one of two things: 1.) completely carbonized build-up that has no grease left in it for lye to be effective, or 2.) an area where rust was once active. The first usually requires steel wool or a SS scrubber and some elbow grease. The second can often be removed or at least minimized by spraying on straight white vinegar, letting it sit a minute, then scrubbing, again with steel wool or a SS scrubber; rinsing and repeating as necessary. If not raised, either type will blend in with additional seasoning/usage.