Anime is known for starring characters with strange or unusual hair colors, with pink, green, blue and other hues being the norm for various show cast members. At the same time, one commonality is that the main characters, particularly in shonen stories, have black hair. Several of the biggest heroes in the medium have dark-colored hair, including the face of shonen anime as a whole.
There are several reasons for this coloration, most of which are practical. From the manga origins of most anime to the country that the stories hail from, there would seem to be more behind the dark tresses of many anime characters than mere coincidence.
Goku, Luffy and Other Anime Heroes All Have Black Hair
Perhaps the most iconic anime character of all is Goku, the hero of the Dragon Ball franchise. Though his various powered forms may sport different hair colors, Goku starts out with black hair. Dark-colored hair is even mentioned by fellow Saiyan Vegeta as being the norm among their race, with Goku's sons Gohan and Goten all sporting hair colors and hairstyles similar to their dad. Goku isn't the only shonen anime hero with this hair color; Luffy, the main character from One Piece, also has black hair. The same goes for many of the Joestar family members in JoJo's Bizarre Adventure -- a series that became a more adult-oriented seinen manga and anime.
Speaking of which, the incredibly violent seinen series Fist of the North Star had a black-haired protagonist in Kenshiro, though his locks were occasionally colored a dark brown. Beyond these demographics, even kids' shows have protagonists with black hair. Nobita Nobi from Doraemon and the buttcheek-boasting Shin-chan from the show of the same name all have the same black hair color, as does Ash/Satoshi of the Pokémon anime. These shows in particular illustrate one of the main reasons for so many anime characters having black hair.
Relatability and Practicality Gives Most Anime Characters Their Black Hair
Japanese people tend to have black to dark brown hair, with the former being the most common. Thus, it's no surprise that their artistic representations would reflect this. As mentioned, many kids' shows in particular have black-haired protagonists, and it's likely so that audiences -- in this case, young boys -- can more easily relate to what the main characters are going through. This extends to the power fantasy and wish fulfillment aspect of shonen stories, as it would likely be easier for the typical Japanese male to "see" themselves as Goku than a more overtly foreign-looking character.
However, this doesn't explain the presence of shonen heroes and other main characters in anime with other hair colors. For instance, Naruto Uzumaki and his son Boruto sport very un-Japanese blond hair. Likewise, characters such as Natsu from Fairy Tail have hair colors that are completely unnatural. This is likely an extreme version of the "fantasy" aspect of anime and manga, resulting in character designs that are more an exaggeration of reality than truly reflective of it. Thus, Naruto's blond hair may be seen as just as unnatural as Natsu's pink tresses.
Another explanation is that black hair shows up more easily in the pages of the original manga that inspire some anime. Though not all anime are adaptations, many of the shonen variety are. Given that manga are typically black and white, hair that's black requires less detail to stand out on white pages. More than likely, it's a combination of these factors -- not to mention, of course, the simple fact that dark hair is simply the norm for many people in general.
