Defender Ridgewell, 23, becomes the first Aston Villa player since 1984 to move to the blue half of the city.
But he insisted: "I came to Birmingham to play football and the fact I've left Villa doesn't come into it.
"I am looking forward to working with Steve. He is a good manager and he has done a lot in his career as a centre-half so that's a lot to look up to.
"I think we should be in for a good season. The manager has invested in the squad and brought in quality players."