the only black grass like plant I can think of is Ophiopogon nigrescens - doesn't exactly seed itself easily, but does spread through root rhizome runners easily, once it's established.

Ophiopogon planiscapus nigrescens, what a lovely mouthful. When it does seed itself about a third of the seedlings are green instead of black.

Above is worth growing cos they cost a fortune in garden centres and once you have some you can keep splitting it. Looks good with contrasting foliage.

I agree,I bought one years ago.seemed expensive at the time,but they split easily,even though they do seem to take a while to get going.They look lovely along the front of a boarder with yellow violas behind,especialy when not much else is looking good in the garden.

Got news for you guys, the price has come down - a friend of mine wanted some and couldn't wait for me to split mine, so we found small pots of it last autumn at £1.99 each.