"I'm absolutely delighted to be sorted for the foreseeable future." said the former Celtic striker who put pen-to-paper on a three-year deal.
Johnstone, who had been on trial in recent weeks is pleased to have finally signed for Blues: "It's a massive club, even when I visited the stadium this morning, it's a great place to play your football and I'm looking forward to the challenge ahead."
"I'll give 100% every game and hopefully get some goals along the way." added the striker.
Watch Denny's full interview below: