The problem with a blind is while you might get the muzzle out the window (which I have done) on a flintlock the vent hole sends out a stream of spark.

Perhaps if you put a chunk of old wool blanket and clothes or safetypinned it to the inside wall. Wool does not burn and should absorb the spark. Plus if it don't you have something to beat the flames out with..

Some of them blinds are pretty narrow. I never shot a flintlock off inside a blind.. sounds like an experience. Perhaps try a shot or two first out of one, with the escape hatch open.. and some items inside for fire control...