The key pair were hurt during an intra-club practice match on Thursday, adding to the Hawks' injury woes ahead of their round-one clash with Essendon on March 16.
Blanck damaged his left knee in a tackle and will miss the entire 2024 campaign, leaving a big hole in Hawthorn's back-line.
The promising 23-year-old has impressed in a key defensive role since being picked up in the 2022 mid-season rookie draft.
"We all feel for James at this point in time following a strong pre-season and what looked to be a promising year ahead," Hawks football manager Rob McCartney said.
"In saying this, James's character will see him quickly turn his attention to being diligent throughout his rehab so he can return to the field in 2025 ready to build on the development we have seen during his first two seasons in the brown and gold.
"As a club, we will now support James as best we can during his recovery phase. We are all feeling for him."
Dashing playmaker Jiath has been slated for a move to the wing this year and impressed in that role during the first half of Thursday's full-length hit-out at Waverley Park.
Changkuoth Jiath exited Hawthorn's intra-club practice match on Thursday with a hamstring injury. (James Ross/AAP PHOTOS)
But the 24-year-old speedster, who has a history of hamstring issues, was hurt during the second half and was immediately sent for scans.
Jiath managed just eight games last season as he battled calf, groin and achilles injuries.
He spent time in Qatar over the off-season working with biomechanics experts in a bid to stay injury-free throughout 2024.
"It's frustrating for CJ. He's done plenty right to get himself to the position he's in today," Hawthorn assistant coach David Hale told reporters.
"He's had a great pre-season and hopefully it's on the minor end of the scale.
"Scans will give us a bit more information but fingers crossed.
"He's a great kid and he's coming back in good nick, so hopefully it's not too long for him."
Hawthorn are already set to be without reigning best-and-fairest winner Will Day for round one as he recovers from a foot stress fracture.
Dylan Moore is also in serious doubt for the Essendon clash after being diagnosed with glandular fever, which will keep him sidelined for a month.
"We're hopeful he can still get up for round one but those things are hard to put a time frame on," Hale said.
"Fingers crossed we can get him going (because) he's a really important player to our structure."
In better news for Hawthorn, emerging youngster Josh Weddle has signed a two-year contract extension.
It ties the 19-year-old speedster to the Hawks until at least the end of 2026.
Weddle played 17 games in his debut season last year after arriving at the Hawks as pick No.18 in the 2022 draft.
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