People ask me this all the time. How do I get such dark backgrounds on my drawings. The answer is complex, so I will step it out below.
1) The paper you use MATTERS! I use only Bristol, which is 2 or 3 times thicker than drawing paper, so it has a strong tooth to the paper
2) I use either B or 2B. The softer you go (3B-9B) the more wax is in the graphite, which makes it like a crayon and hard to blend.
3) I KEEP the pencil SHARP, and I hold it SIDEWAYS.
4) I do small sections at a time
5) I TURN the pencil in my fingers as I darken the area. This also helps keep the pencil sharp.
6) I use a back and forth scraping motion, that I can FEEL dig into the paper.
7) I end up going back over it more than once before finished, in a different direction.
8) I LATER use a harder pencil (H, F) and go over the dark area again
9) I use a "workable spray fixative" made by Krylon which reduces glare
10) I never use a flash on the camera when taking the photo of it
11) I ONLY take the photo during the daytime, under natural light, by laying it on the carpet near a window.
12) I NEVER use a software program to darken the photo of the drawing
I hope this helps?