How to select edge/ black lines in an image only?

[Ofnuts]

After the median blur:

• Fuzzy select the background
  
• Select ➤ grow by one pixel
  
• Select ➤ shrink by one pixel  (you can try the same grow/shrink with 2px instead)
  
• Select ➤ Invert
  
• Adda layer, filled with white
  
• Bucket-fill selection with black