The Australian right back was cautioned for 'diving' under a challenge from Mikael Silvestre but Rennie has now decided to overturn the decision after reviewing video footage of the match.
Neill would have missed the all-important second leg of the semi-final if the yellow card had stood and manager Graeme Souness was delighted to have the defensive pillar back for the return leg at Ewood Park.
"I am absolutely delighted with the decision," Souness told the club website.
"I was always quietly confident because Uriah is one of the better ones around and he was looking at the incident from behind.
"He couldn't have seen that Lucas had got inside his opponent. It would have been difficult for him to see what really happened.
"I felt that if he looked at it again, then he would see it how we saw it, we are all very pleased."
