Slughorn most likely didn’t know the Dark Mark’s incantation.
The correct incantation to conjure the Dark Mark was something that most likely only Death Eaters know, which is what Arthur Weasley kept trying to remind Amos Diggory. Crouch was blustering and acted quite offended at the suggestion that Winky cast the Dark Mark - but he would have suspected it was his son he’d smuggled in, and be trying to throw suspicion off himself.
“You’ve been caught red-handed, elf!’ Mr Diggory roared. ‘Caught with the guilty wand in your hand!’
‘Amos,’ said Mr Weasley loudly, ‘think about it … precious few wizards know how to do that spell … where would she have learnt it?’
‘Perhaps Amos is suggesting,’ said Mr Crouch, cold anger in every syllable, ‘that I routinely teach my servants to conjure the Dark Mark?”
- Harry Potter and the Goblet of Fire, Chapter 11 (Aboard the Hogwarts Express)
Arthur Weasley, who also worked for the Ministry and had no ulterior motivation, said it’d be highly unlikely that the person who’d conjured it would have never been a Death Eater.
“But I’ll tell you this … it was only the Death Eaters who ever knew how to conjure it. I’d be very surprised if the person who did it hadn’t been a Death Eater once, even if they’re not now … Listen, it’s very late, and if your mother hears what’s happened she’ll be worried sick. We’ll get a few more hours’ sleep and then try and get an early Portkey out of here.”
- Harry Potter and the Goblet of Fire, Chapter 11 (Aboard the Hogwarts Express)
He was proven right about this when it turned out to be Barty Crouch Jr. who’d conjured the Dark Mark - he was indeed a Death Eater. So, it seems highly unlikely that Slughorn, who wasn’t one and kept away from war, would have known the right incantation to cast the Dark Mark.
He could, however, have attempted to make a fake one.
While only Death Eaters knew the correct incantation to conjure the Dark Mark, other wizards knew what it looked like, so Slughorn could have attempted to “forge” it. He’d used dragon blood to fake human blood, so he could have tried something similar. Fred and George were able to make candy Dark Marks, presumably without knowledge of how the real Dark Mark was cast.
“Come through to the back, Harry, that’s where we’re making the real money – pocket anything, you, and you’ll pay in more than Galleons!’ he added warningly to a small boy who hastily whipped his hand out of the tub labelled: Edible Dark Marks – They’ll Make Anyone Sick!”
- Harry Potter and the Half-Blood Prince, Chapter 6 (Draco’s Detour)
Should he have thought of it, Slughorn might have tried replicating the Dark Mark - it might not be indistinguishable from the real thing, but he’d likely have tried to make something like it.
“Your guess is as good as ours, Hermione,” said Mr. Weasley. “But I’ll tell you this . . . **it was only the Death Eaters who ever knew how to conjure it.** I’d be very surprised if the person who did it hadn’t been a Death Eater once, even if they’re not now. . . .
– Gilles V. Dec 12 '14 at 15:44