Dark blue concrete wall with some small dust spots.
Source Atle Mo
Remixed from a design seen on Pixabay. The basic tile can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Sounds like something from World of Warcraft. Has to be good.
Source Tony Kinard
This could be a hippy vintage wallpaper.
Source Tileable Patterns
Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
Prismatic Groovy Concentric Background
Source GDJ
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
Prismatic Rounded Squares Grid 4 No Background
Source GDJ
Remixed from a JPG that was uploaded to Pixabay by Osckar
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten