The 26-year-old will formally join Blues at the start of July on a two-year deal after his current contract with the SPL outfit expires.
"Darren was somebody we monitored for a very long time, so we were delighted we were able to bring him in," said Clark.
"He had a fantastic time up in Scotland and he’s still young. He obviously had other offers, certainly from other Championship clubs and potentially clubs higher, but we're happy he chose to come here.
"With the re-signing of Colin (Doyle) it means we’re very strong in the goalkeeping department really early in the summer, so that’s one area we can leave now.”
Whilst the Blues boss is delighted to bring in a player of Randolph's quality, he insists that Doyle (pictured below)has not been ruled out of the equation.
"Darren hasn’t received a guarantee that he'll start," added Clark.
"I said the same to both him and Colin that I want what you would class as two first team goalkeepers and it’s up to them really, and then for me to decide.
"Basically they’ll be given an opportunity over the course of pre-season and then we’ll go from there. Whereas outfield players have an opportunity to play in different positions unfortunately they are committed to just one, even though Colin kept saying to me he could play as a striker especially during that spell last mid-season when we were without any recognised strikers!
"I’m now really settled with the two lads we've got and there is also young Nick Townsend who is coming through and doing extremely well for us."