This is the topic to discuss how you blackwash your customs.
I have two strategies.
I have a bottle of extremely thinned matte black water soluble acrylic. I coat the figures with it, and drip water to make the paint run off of the figure. The paint settles in the seam lines and details.
My other strategy just imitates a blackwash when done. I mostly just use it on larger figures. I coat whatever part I am working on entirely in matte black. I then wipe off as much of the paint as I can, leaving the details shadowed and the whole part slightly darkened. The last figure I did this to was a Halo Mattel Jerome figure.