The Ospreys, unbeaten against a fellow region at the Liberty Stadium since 2010, trailed 6-0 after 50 minutes as two Jarrod Evans penalties put the Blues ahead.
But Davies, who came on at the interval after Dan Biggar suffered a back spasm, took control of the game as his pin-point kicking from hand pinned the visitors in their own half for long periods.
The Ospreys profited with captain Alun Wyn Jones scoring the only try of the game and Davies adding eight points with the boot for a final score of 13-6.
Rees believes Davies, who was World Junior Player of the Year in 2013, is showing the benefit of his close working relationship with Wales international Biggar.
“Sam had a bit of a tricky start but we all know what a quality player he is and he grew into the very well and put us in some good positions,” said Rees.
“He took on the half-time messages about where we could find ground through the kicking game, about building pressure, not getting worried by the scoreboard and he did that for us.
“He and Dan are good together, he is learning from one of the best in the business, they spend a lot of time together and problem solve together. For Sam it is about the balance of how to impose himself on a game and he certainly did that.”
