Danny Haynes had put Ipswich ahead early in the second half after Matt Richards had played him through.
Richards and Ian Westlake both missed the target for Ipswich while Haynes struck the crossbar.
Leeds winger Eddie Lewis shot wide, while striker Rob Hulse forced a good save from Shane Supple, who also did well to deny Liam Miller.
"Firstly it wasn't a corner and then Rob Hulse banged into Jason, which is why his arm went up with the impact.
"I've seen it three times on the video now and there is no doubt.
"How the referee saw the handball but didn't see the foul on De Vos is amazing. He said he didn't see it so there is no more to be said."
"We came to Ipswich at a time when they are full of confidence and we were not quite on our game," said Blackwell.
"Ipswich is a terrific place to play.
"The young Ipswich goalkeeper made two unbelievable saves which really inspired his team.
"Ipswich had good spells but we stayed unbeaten which was important."
Ipswich: Supple, Wilnis, Richards, De Vos, Naylor, Magilton (Currie 45), Garvan, Barron, Haynes (Proudlock 68), Lee, Westlake.
Subs Not Used: Price, McDonald, Sito.
Booked: Garvan.
Goals: Haynes 48.
Leeds: Sullivan, Kelly, Crainey, Butler, Gregan, Hulse, Lewis, Miller (Healy 57), Douglas, Derry, Cresswell.
Subs Not Used: Bennett, Pugh, Walton, Blake.
Booked: Gregan, Healy.
Goals: Healy 88 pen.
Att: 25,845.
Ref: P Crossley (Kent).