Thermal printers make black marks with heat, and the darkness (density) setting in Settings → Printers controls how much heat is used. If barcodes look faded, gray, or speckled, raise it a step or two and print a test label.
Stop at the lowest setting that gives solid, crisp black. Going higher doesn't improve scanning. It shortens print head life, uses more power, and can make barcodes bleed and become harder to scan.
If labels are still faint at a high setting, check the other causes first: dirty print heads (clean with isopropyl alcohol), old or low-quality label stock, or direct thermal labels that have been exposed to heat or sunlight. If you still have problems, consider replacing the printer's power supply cord; some aftermarket power supplies don't provide enough power for reliable printing.