Leeds prevented Watford's in-form midfielder Matthew Spring from playing against them through an agreement when he moved from Elland Road to Vicarage Road in August, so 18-year-old Alhassan Bangura filled the vacancy.
The youngster from Sierra Leone wasted a good chance when he headed straight at Neil Sullivan after Leeds failed to clear a 24th minute corner, but Blake was full of ideas in the Leeds attack and he deserved better after deceiving two defenders and seeing his neat chip palmed over by Foster.
Central midfielders James Chambers and Bangura had to watch their step after collecting early bookings, but Watford took a 40th minute lead when Sean Gregan brought down Marlon King inside the D and Young curled a perfect free-kick into the top left corner, giving Sullivan no chance.
Eddie Lewis and Gregan were denied by excellent saves from Foster just before the break.
Both sides had goals disallowed for offside in rapid succession, Blake in the 47th minute and Henderson two minutes later, but Blake had his reward with a 59th minute equaliser. Jordan Stewart's back pass was too short and Foster brought down Rob Hulse and was red-carded.
Chamberlain left the bench to take over in goal, Blake beat him from the spot and Chris Eagles was substituted.
Worse followed for Watford two minutes later when Henderson foolishly elbowed Gregan and followed Foster for an early bath.
Leeds went for the jugular, sending on David Healy and Richard Cresswell in place of Jonathan Douglas and Hulse with 15 minutes remaining.
But it was Blake who broke the nine men's resistance on 81 minutes after Cresswell broke free on the left, Healy's shot was parried by Chamberlain and Blake fired home the rebound.