In the wonderful world of non-live action porn, there is a tendency for pubic hair not to be darker than the hair color of the person in question, which sometimes are victims of You Gotta Have Blue Hair
, thus you get the tendency of having things like blue or blonde pubic hair.
The reasons this happens is multi-fold. It could be Did Not Do The Research
, especially in the case of Japanese-originated works of non-live-action pornography, where light-haired people are not as common as dark-haired people, thus the pubic hair usually matches the hair color of the person in question in real life in the author's home country.
It could be The Coconut Effect
, where mismatched head and pubic hair color looks "unrealistic", even if the author is aware of the usual mismatch in color. Related but different, it could be Artistic License
, where the author (and/or audience) find matching pubic and head hair color appealing.
Also note, this can be a Justified Trope
for any of the above reasons, as people in real life do dye their pubic hair to match their head hair, which could also be dyed a unique color like blue.
[If this trope doesn't meet TV Trope's wanted level of appropriateness, I will move this proposal to Fetish Fuel Wiki.]