it's a trend i guess, previously they used to have blonde bottoms and black haired tops (dangerous convenience store, on and off, love is an illusion, etc) but then the trend of blonde tops and black hair ukes emerged, idk which manhwa started it, but id like to think it was shutline(i can be wrong tho) but it's the manhwa which went super popular for a mafia theme, a buff bottom and a blonde top.
idk if its just me but most of the blonde hair tops are foreign/non korean residents so i guess its an attempt to make them look westernised (aaron from flashlight, seo in from backlight)
Why in every bl manhwa the top is blonde and the bottom is black haired, just do something new we can't now memorize the titles of manhwas cause of that!!