Denis and his wife, Elaine, were holidaying at the beach near Sousse when they were killed.
Denis was a talented professional at Blues between 1962 - 1972 and he was part of the squad that won the Club's first major honour, the 1963 League Cup.
Born in the north east town of Stockton-on-Tees in December 1944, Denis' footballing ability shown through from an early age and he was selected to represent England at schoolboy level.
He initially joined Blues on amateur forms in 1960, turning professional two years later by which time he had already made his first team breakthrough at St. Andrew's.
A pacy outside left, Denis made his debut at the age of just 16 in a League Cup tie at Swindon Town in September 1961, which Blues lost 2-0.
His league bow came almost 12 months later at Highbury in the second game of the 1962/63 season, a 2-0 First Division defeat to Arsenal.
He replaced an unhappy Bertie Auld, who promptly handed in a transfer request, and went on to feature in six of the opening eight league matches of that campaign.
But when Auld was recalled and rediscovered his form, Denis returned to reserve team football and was a spectator as the team clinched the League Cup trophy in 1963.
For three seasons Denis mainly played second fiddle to Auld, figuring only occasionally in the first team, but still managed to notch his first two club goals in a 4-3 home defeat to Chelsea in March 1964.
When Auld returned to former club Celtic, Denis got more of an opportunity to shine and missed only six games between November 1964 and April 1966.
Denis' appearances became less frequent during the latter stages of his Blues career.
One of his last games in February 1969 was one of his most memorable though.
He helped Second Division Blues hold an illustrious Manchester United side, including the likes of George Best, Bobby Charlton and Denis Law, to a 2-2 draw in an FA Cup tie in front of over 50,000 at St. Andrew's.
Denis retired in 1972 at the age of 27 but did pull his boots back on to turn out for Rover Solihull in the Birmingham Works League.
He later moved with his family to Blackpool, where he worked as a hospital porter.
Ex-team-mate Garry Pendrey has been left stunned by the news of Denis' death.
"It is unbelievable," said the former Blues stalwart captain and manager.
"He was the equivalent of Trevor Francis in his time, he really was.
"Denis was a terrific player, he had tremendous ability. He could go past people, score goals.
"Unfortunately he wasn't really able to reproduce his talents on a consistent basis, he had a problem with nerves.
"But he was some player, and a lovely, lovely man."
Denis' father, Jim, also worked at Blues on the St. Andrew's groundstaff.