Burnt/dried residue?

So a few weeks ago, we did a batch of canned spaghetti sauce. The sauce did touch the lid while boiling (should probably have left more head space) but the seal is good, I can easily lift the jar without it coming off.

The problem we have is that there seems to be some kind of dried residue, but it's a bit blackish could it be burnt sugar or could it be mold?