I noticed (a bit too late) that a second hand striped white shirt I bought in a charity had a small dark burn mark - see picture. Is there any way to hide it or remove it?
If it is the fibres that are burnt you can do little to remove the burn, anything you do will wear it out if the fibres are already damaged.
A careful hand wash with extra attention to the spot may dislodge any burnt jam or other dirt.
Using a chlorine or peroxide bleach may save the day if it is an organic stain that looks like a burn. Strong bleaches will weaken fabric and may strip the colour from the nearby details so test out of sight first.
Using a white marker carefully (check in a hidden spot first) may just be able to make your spot the same colour as the shirt but not remove it.