I don't have any suggestions for cover up. When I cut myself in the past, I didn't bother trying to cover it up because all the excuses seemed stupid. And frankly, I couldn't be bothered with what others thought. And even if I did know cover-up strategies...I probably wouldn't share them because that might make it seem easier to cut, knowing that there are ways to hide it. =/

I hope you decide to stop self-harming soon.