While I can't dispute the value (pun intended) of the comments about pushing contrast, I also know that the normal graphite pencil used in school goes only so dark as opposed to charcoal pencils and so on. I remember in rendering classes that we were to use graphite. But the brave among us got into using workable fixative which essentially aerosol sprays from a can a little clear varnish like layer that when dry allows you to put more layers over the old graphite. It's stuff you generally don't want to breathe. Some odorless hair sprays work as well. But you would want to test it to see if it turns your paper with a yellow brown stain.
And if you get soft enough graphite, it will eventually get dark, though as I recall it never got black. Graphite is great to work with though even though it didn't go black. I used to love it probably because it was more of a gray and I could then paint over it with thin washes in some cases, letting the line show through. It was a popular illustrator style back when I was getting into the business in the 70s.
Anyway, that value thing is emblematic of the medium. And if you want to go for black, you need perhaps a different mark maker like charcoal, conte crayon or prisma color pencils etc. You'll have a blast exploring the different tools for mark making.
And yes. Contrast is usually good. Problem is that if you make a mistake it's pretty apparent. And that's why many people like graphite. Plus it is usually pretty easily erasable. But on the other hand, if you know that mistakes will loom large, you may be spurred on to draw better and better so it's no longer an issue. People who draw in pen and ink tend to come into a sureness with their marks owing to the unforgiving quality of the black ink on white paper.
Okay, having said that, use whatever tools you can latch onto. And there's nothing wrong with the ol' #2 pencil in a sketchbook. The important thing is to keep drawing. You have been. And it looks great. I personally love the more whimsical stuff because I still doodle like that. My mind just sort of spills it out when I have pencil in hand during a lecture or something. And often times the images really do tell me what is rolling around in the back of my head and sort of symbolically represents those impressions from the lecture for example. Kinda cool.
Anyway, I agree with the others. And you're doing great. Keep up the wonderful work.
"Not a bit is wasted and the best is yet to come. . ." -- remembered from a dream