It's called drywall.

Frame around it and cover it with drywall.

OK, 2 years later I find myself in the same boat with the exact same duct work situation in basement. Framing out and putting up drywall is both costly and intrusive into the space. What did you do with yours? My hvac person suggests painting it. I looked into installing the ultra cool round industrial looking ducts, but it is SO expensive. Thank you!