Somerset were in trouble as soon as Jayasuriya went to the 10th ball of the day, following a hint of Willoughby swing to be caught at first slip by Darren Maddy for a duck off a defensive outside edge.
Trescothick stayed long enough to take advantage of Charlie Dagnall's inability to find his line, picking up two leg-side boundaries before he became the first of Willoughby's lbw victims to a full-length delivery which trapped him on the crease.
The next three dismissals were action-replay variations on a theme - Keith Parsons, James Hildreth and John Francis all undone by the same recipe of full-length late swing and umpire David Constant sending them on their way.
Willoughby was repeating the heroics performed yesterday by his compatriot Charl Pietersen, whose seven for 10 for Northamptonshire against C&G newcomers Denmark in Copenhagen was the second-best return in this competition's history.
Leicestershire's new overseas signing completed his wickets column when Aaron Laraman clipped a routine catch to square leg from another full-length delivery.
Then Ottis Gibson (three for 22) collected the fifth of six lbw verdicts in the innings, Rob Turner pushing forward and missing one which nipped back and kept low.
Gibson pulled off a low return catch to shift Richard Johnson and snapped up his third wicket when Andrew Caddick was gobbled up at second slip.
From 58 for nine, Blackwell had no option but to plunder what he could before the inevitable last wicket arrived.
He managed two driven boundaries down the ground off Maddy's medium-pace and a huge swept six off Claude Henderson but he too went lbw, his departure the slowest of the day as he eventually tore himself from the crease with an incredulous stare at Constant's decision after he had pushed forward and missed the slow left-armer.
Overhead conditions eased for Leicestershire's batsmen but with swing still available in plentiful supply and some help off the pitch as well, it remained far from plain sailing.
Maddy fell to a sharp caught and bowled by Simon Francis (three for 32); John Sadler offered no shot to be very lbw to Johnson's inswing and Dinesh Mongia played on involuntarily as Leicestershire went from 18 without loss to 19 for three.
Aftab Habib and Hylton Ackerman's rescue work faltered when the former played across Caddick to be yet another lbw statistic and his captain flat-batted the same bowler straight to cover.
After Jeremy Snape had also paid for pushing across Johnson, there was much work to be done from 53 for six.
But Paul Nixon and Gibson met the challenge with some much-needed good sense and firm blows when the opportunity arose to get their team close to the finishing line.
The wicketkeeper nicked one behind off Francis with 25 still needed before Gibson and Henderson eked out the remaining runs required.
