Brian Howard cancelled out Sean St Ledger's first-half opener for the home side.
"Our performance over 90 minutes was outstanding in a stadium that's quite hostile," said Blackwell.
"Having gone 1-0 down, I think quite a few clubs would have crumbled but the players showed such mental strength. There were five players under the age of 21 here and they're starting to grow.
"I think you can always want more but if you had said to me when I got on the bus at Bramall Lane this morning that we'd come back with a 1-1 draw I'd have been quite happy."
The Preston North End boss Alan Irvine insisted his side still have "every opportunity" of reaching the final at Wembley.
"There's no doubt that if we play well enough at Bramall Lane we can win, we're still very much in this," he said.
"Obviously it would be nice to be going there with a lead, but it's not like a European game where away goals count double and you're going to an unusual venue.