Trying to get my darks darker

What sort of materials are you using? 

You may want to try something more opaque like a marker, fine liner, smaller piece of charcoal, a 6B pencil, so forth. Something with more control and also applying it after everything has dried/making sure its not going to be moved around.

Hi Zacrim! Toda raba, it’s a nice idea, I will do so and post the changed pic. Have a nice day!