With a book open, tap the top of the screen, then tap Page Display, then Font & Page Settings. You can adjust the Bold setting there. You might also try different fonts, as some are darker than others.
I tried both adjusting the boldness and changing the fonts, but the font color still looks greyish instead of black, I personally like dark black text color while reading, isn't there any way where we can make it look darker ?
With the new feature of having customized fonts, can we adjust the font color in any way?
being that a Kindle screen is basically like an etch-o-sketch, reducing the illumination on it, reduces the illumination shining down on both the background and the font. On my Voyage and paperwhite, the blackness is the same, what is reduced is not the blackness, but the contrast between the background and the font so the font appears to be lighter/muted