He scored his first goal when he converted a second half penalty but the occasion was spoiled when he was dismissed for two bookable offences.
Blackwell heaped praise on the club's latest protege and is tipping him to follow in the footsteps of past heroes Alan Smith and James Milner.
Said Blackwell in the Yorkshire Evening Post: "I don't want to put pressure on him, but the fans are looking for a new idol. He may only be 16 but he showed me more than some of the trialists we've had with 20 years experience.
"If he can perform and develop like Alan Smith and James Milner there's no reason why he can't go on to become a cult figure at Elland Road. We have a precocious talent there and my job is to make sure the kid develops as a player.
"He said at half-time he wanted to take a penalty and he took it well. Watching him run across to the Kop with his arm raised a-la Smithy was marvellous and hopefully in time he will be as well received as Alan was."
Walton was disappointed to see his afternoon abruptly ended when referee Matt Messias awarded a second yellow card following a lunge on Lebero Parri.
The incident provoked a typical fracas with all 11 Spaniards racing in to create a needless confrontation. Blackwell added: "Simon is one of the quietest lads you can meet and the tackle was just exuberance. Matt Messias is a respected referee and I thought he might have let it go, particularly given the nature of the game.
"Worse challenges than that went unpunished, but maybe the reaction of the Valencia players made it look worse than it was."
Blackwell was also pleased with the team's overall effort. Julian Joachim and Danny Pugh caught the eye, Matthew Spring and Frazer Richardson were impressive in midfield, and the back four is starting to have a good look about it.
"It was a great exercise, not least because a lot of the lads haven't even played here before," said Blackwell. "I was also heartened by the crowd and it was pleasing to send them home after watching a team that gave their all."