You may want to give a bit more context. When reading the headline I thought you meant in humans as opposed to in other animals. When reading the text, I got confused. Very. I don't know whether to answer from a biology or artistic framework.

What is blonde? As a hair color in humans, you could say we are all pretty much similar in hair color, the difference being the amount of melanin in the hair strands. You have the red melanin type, and the brown melanin type. Typically most people have a combination of both. The more melanin, the darker it gets. Blondes would be those with a medium to low concentration of melanin in their hair.

You will also find the idea of what consists of a blonde shade or not is also subject to cultural standards. But just to play it safe let's agree we are talking about the paler shades of blonde. It doesn't really matter. It's always a light ochre shade, sometimes slightly more or less red (leaning more towards orange, otherwise to white).

Lots of animals have hair in a similar tone, see golden labradors or palomino horses for example.

Artists have been accurately depicting blonde hair in a somewhat realistic manner for centuries, I am sure you have seen paintings with people of blonde hair in them that are centuries old.

When it comes to cartoons, you can't afford to paint every frame like an oil painting. The time and cost would go through the roof. This is one reason cartoons are not realistic. (3D rendering can recreate blonde shading realistically at an affordable rate though).

Another reason, and probably the main reason I would argue, is that cartoons are not meant to be realistic. Stylization is on purpose. Stylization and abstraction open up a universe of possibilities for artists to express themselves and make visual gags and things impossibly beautiful, ugly, interesting.

So either because of stylization or simplification or both, artists have narrowed down hair color to a single tone (sometimes two tones or three if with highlights and shadows like some animes do) per character, and so for blondes they choose tints and shades of yellows and ochres. It's the same really, just reduced to its essence.

Now as to what makes it impossible to create a blonde marker or pen: the same reason you can't create a marker that automatically shades to perfection whatever you are painting.

Say you have a white sphere. You will never see it as flat white, you will see greys where the environment casts a shadow over it. The shadows will be different depending on the light conditions. To make it worse, you will see any color in the environment reflected indirectly on the sphere as light bounces and spills everywhere. If the sky is blue, your sphere now has traces of blue in it. If the ground is red, you now also have traces of red added. And so on.

The same happens to hair. To make it more complicated, hair is somewhat translucent so you now have to take into account how the light travels through and reflects and refracts when it hits every strand of hair. Plus, not every single strand of hair is exactly the same shade. So it's impossible to make a "blonde" paint. You can paint an approximation of blonde the same way you can paint an approximation of light, shade, reflection, translucency, etc on any object. You mix different pigments and recreate the illusion, but you can't create a pigment that automatically adjusts to arbitrary and unpredictable standards, if that makes any sense.

Blond hair isn't a single color. There are lighter and darker individual strands that make up the whole.

What single color would you pick? Give us a hex code.

Brother, what are you talking about?

There's even some fully hand animated and colored stuff from Disney.

There's blonde beer (even in several languages)

Camels, golden receivers

edit: retrievers

In addition to the other answers, animation isn't just about depicting things as they are. Its a language that uses color and symbols to communicate cultural ideas. Sometimes the hair is drawn as yellow instead of blonde because their are emotions associated with it that the artist wants projected on to the character

There's only really two honest answers to this:

1. you're a troll

2. you need an ophthalmologist

Because hair color isn't homogeneous. Every hair is a slightly different color, and it's significantly more noticeable in lighter hair. Blonde hairs are predominantly gold, but range from white to brown. In cartoons, gold is also usually just yellow, because animating something metallic is a pain in the ass. Hair is shiny as well, so they picked the average color and called it good enough. Grey hair is the same way.

As for only existing in humans, that's just not true. Yellow labs and golden retrievers come to mind immediately, and plenty of rodents are blonde. While those are fur, not necessarily hair, golden doodles have hair, and are usually some manner of blonde. Golden macaques, flying squirrels, lions, etc.

Horses.

Johnstones Trade Smooth Metal Paint (Tinted) 0.8L - True Blonde

https://www.brewers.co.uk/product/AF12506690N/ppg12-12-true-blonde

My blonde hair kept always confusing small boys. Are your hair yellow or white?

So, there is that, blonde is not a color, it's a whole spectrum of colors from white, yellow and brown.

Also beer