* Genetics: It's uncommon to have natural blonde highlights or streaks in predominantly black hair unless there's a genetic reason for it. This could be due to a recessive gene for blonde hair expressing itself in a small area or mosaicism.
* Sun exposure: In some cases, prolonged sun exposure can lighten certain sections of hair, particularly on the sides where it's more exposed. This is more likely to result in reddish or brownish tones rather than blonde, though it's possible.
* Chemical treatments: The most likely reason for blonde streaks in black hair is intentional dyeing or bleaching.
Overall:
If the blonde is natural, it's relatively rare. If it's caused by sun exposure, it's more common but the shade might not be a distinct blonde. If it's dyed, it's as common as people choosing to dye their hair that way.