I am wondering, what makes us have a personal preference in a sexual sense.

Like, some dudes prefer girls with blonde hair, others prefer girls with black hair.
Or, some dudes prefer dark skin, others prefer light skin.

What makes us have preferences like this? How does it originate?

I've heard a couple of things from different people during my life...

The first one is that how your parents look is an important factor that determine the qualities you become attracted to. If you've had a good relation to your mother as a kid, you probably prefer women with the same hair color, the same body type etc. It sounds a bit incestuous, doesn't it? On the other hand, if you hate your mother for some reason, you may prefer women that look as different as possible. So if your mother is white, you may prefer black women. I don't know if all this is true, but I think we can agree that it is quite complex.

The second thing I've heard is that some people, those who have a strong need of feeling safe, are attracted to those who look like themselves. If you already feel safe withing yourself, you don't care as much about that.

Like, we are all attracted to a variety regardless. But, we have different levels of interest relating to certain qualities a girl has. I suppose, the same applies for girls when they think about men.

The reason I am asking this is because; one, I am a sceptic and two, I personally seem MORE attracted to black girls than I do to other 'types' of girls, in a physical sense at least.

my theory is simple think about what you are typically attracted to. then think back to when you were young and you pined or liked a girl but it was never mutual or you never did anything about it. Now try and remember what that girl looked like without looking at a picture. I bet what comes to your mind is mainly colors and features.

I feel atleast for myself thats the root cause of what im attracted to

Scarcity is the root of want.
If you haven't been loved enough, you want a very nice lovely girl.
If you haven't had enough boobs, you want a busty girl.
If your mother didn't cook, you want a woman who cooks.
If your sister was brunette, you want a blonde.

Am I right, or am I talking bananas?

It does make sense a bit, as I have never had a black girlfriend and I don't see black girls all the time. But, I am looking for a more scientific explanation for it.

See the thing is is there is not a scientific explanation to this. Personal preference in women is a structural sociological phenomenon. People always think its psychological, whether you believe it or not, society and your general surroundings influence your preferences. Its not as simple as scarcity.