I was going to suggest you try the Perennials Forum for suggestions, but I see you've already posted there.

The only grass-like plant which comes to mind is Ophiopogon planiscapus 'Nigrescens', AKA Black Mondo Grass. I've only grown it in full sun, but it can supposedly handle part shade. Your zone is unknown, so I can't say it'll work for you. It's rated to zone 6.

I was primarily thinking of grass for fluffy qualities ... what about the Eriophorums?

I'm not sure what zone you are in, but purple fountain grass has dark enough spades to look almost black. Also, though they are smooth, there is bugle weed mahogany and a black looking new zealand flax ....

Any of the fountain grasses would qualify under your fluffy criteria.
Pennisetum 'Moudry' even has black'ish coloured plumes...