I'd solo it no bother, I might wind my neck in just a little though. Just treat it as being an XC route in the country, since that's basically what it is. The usual "Glentress is like a road, you could ride it on a shopper" crowd might be along in a second but there's some truth to that it's not technically hard, or especially risky. But IMO it's possible to hurt yourself pretty much anywhere with the right go-getting attitude 😉 It'd hardly be the first time someone's been helicoptered off the black.
Though TBH I'd also say to do the red not the black, it's a brilliant fun trail, very approachable, the black's well worth the ride but the red keeps drawing me back for one more go. And it's easy to cut it up into bits, redo sections etc.