Is it illegal for a convicted felon to possess a gun that only fires blanks?

NO, there should be a law stating that a felon should not be in possession of a firearm,If you are a convicted felon you should know that, firing one proves you were in possession.

No.

I can find nothing in my research that expressly forbids it but the best thing you can do is contact your local law enforcement agency - because - my research did disclose many local jurisdictions...

Since flare guns are used in an emergency, the possesion of a flare gun is legal by a felon if used properly.

Absolutely!!!!