Comeau had a career year in 2010-11, recording 24 goals and 22 assists in 77 games with the Islanders.
But Comeau had zero points in 16 games for the Islanders this season.
"Blake is a smart, committed, two-way hockey player who kills penalties, is responsible defensively and can score some goals as well," said Flames general manager Jay Feaster. "We have liked this player for a long time, and our scouts and staff are convinced a change is going to be good for him. We are excited to add him to our club without having to give up an asset."
The 25-year-old has played in 261 career games, all with the Islanders, recording 56 goals and 65 assists.