How do I keep lint off when I wash black knitted pants?

Make sure you wash with things that don't give off lint like jeans and tun your pants inside out use liquid softener as well and then let then dry outside of dryer. Hang up in shower if you don't have a drying rack. I prefer hanging them up in shower. I do this because no mater how clean you think filter is in dryer it's never enough and lint still lives their.

Always be very careful about cleaning out the filter

try turning them inside out..

If you are ready to trash, fine sandpaper will work

I can almost guess the pants are polyester. That fabric seems to pick up lots of lint, and can pill like crazy too. The above hints will help. But next time you are out shopping, get polyester blends with cotton or rayon. Fibers are smoother and longer so they do not pick up as much lint.

Pin them inside a pillowcase.