Ironically, the number was left vacant at The Hawthorns due to Pablo Ibanez moving in the opposite direction to City in the summer.
"Six has always been my favourite number and at Blues I was delighted to get that number," Ridgwell told the WBA website.
"When I came here and was offered four, six or eight, it was an easy decision for me. I couldn't have it at Villa because that was Gareth Barry's number and he's a Villa great.
"I've got many tattoos and I thought I'd take up a bit of room with a No.6."