Prusa I3 - Burn marks on print
I have seen this after I reassemble the extruder heat block and then leave it too loose during the print.

Is your extruder above the nozzle coated in a brown or black film?

What happens is the liquid plastic oozes through the loose connection, travels around the outside of the block, where it is heated longer and hotter than normal turning it black/brown. Then it drips, or flakes, into the print where it is spread around giving it a diffused look that you see.

The fix is to tighten the heating block. Do this while it is at operating temperature (be careful!) because it fits differently when cold.

Another cause: I made a tiny sharpie mark on my filament.  I did not realize how concentrated it would come out in the print.  Perhaps you have ink or adhesive or something on your filament that you can't see that turns brown when heated.